Why was Lord Shiva always high on marijuana?

Version 1:-

The answer lies in “Samudra Manthan” - Churning of the Cosmic Ocean.

The Dev (gods) and Rakshashas (demons) got together for the colossal task of churning the cosmic ocean. Mount Mandara, was used as the pole to stir the waters. And Vishnu’s Koorma Avatar(Tortoise) balanced the mountain on its back thereby preventing it from sinking in the depths of the unfathomable ocean. The great serpent Vasuki was used as the churning rope. As the ocean was churned lot of goodies came out of it which the Devs and Rakshashas distributed amongst themselves. But from the depths of the ocean also came out the “Kalkoot” visha(poison). When the poison was taken out, it started heating up the cosmos considerably. Such was its heat that people started running in dread, animals started dying and plants started withering. The “Visha” had no taker hence Shiva came to everyone’s rescue and he drank the Visha. But, he did not swallow it. He kept the poison in his throat. Since then, Shiva's throat became blue, and he came to be known as Neelkantha or the blue-throated one.

Now this caused immense heat and Shiva started getting restless. A restless Shiva is not a good omen. Hence the gods undertook the task of cooling Shiva off. According to one of the legends Chandra Dev (Moon god) made Shiva’s hair his abode to cool him. Some legends even claim that Shiva moved to Kailash (which has subzero temperatures all year round) post the Samudra Manthan episode. Shiva’s head was covered with “Bilva Patra”. So you see everything was being done to cool Shiva off
Now coming back to the question – Marijuana is supposed to be a coolant. It lowers down the body’s metabolism and that brings the overall body temperature down. Same is the case with Cannabis (Bhang) and Datura. Bhang and Datura are closely associated with Shiva as well.
Hence summing everything up, we can say – Shiva smokes marijuana to keep his body temperature down. Once the temperature rises, he may transform into Hulk from Dr. Banner and that certainly won’t be a great sight.

Version 2:-

It seems that there is a recurring public outrage against the mere idea of our Gods having fun. Wonder why that is.

It is true that recreational use of psychedelics and other drugs have been associated with spiritualism for thousands of years. But if we specifically speak of Shiva, it must be clarified that there is no textual evidence associating him with recreational use of cannabis. 

People often conflate the behaviour of Gods with that of their most ardent devotees. And it turns out that a lot of yogis and aghoris and even lay devotees have used cannabis for a long time.

Either way, the evolution of the portraiture of Shiva can be seen to undergo certain stages:

  1. Since ancient times: half-closed eyesardhanimīlitākṣa, which many nowinterpret as a sign of a drug-induced "high"
  2. Since medieval times: consumer of bhaṅgā only (Cannabis as mentioned in Atharva Veda and subsequent texts was consumed in beverage form only, and not smoked)
  3. Very recently: as a smoker of charas in chillum form (preparation of cannabis resin)
There are some key parts to breaking down this portraiture

  1. Shiva is both destruction and knowledge personified
  2. In the Mahabharata, we learn that Shiva and Parvati jointly invented gambling with dicean activity noted in-story for requiring knowledge of counting(combinatorics), recreation and addiction
  3. Primary effects of smoking/ingesting cannabis are hallucinations (knowledge?) and euphoria (recreation/ananda?)
  4. Ayurvedic texts such as Rasaratna Samuchaya were also already aware of thesemi-toxic (destruction?) properties of cannabis
Simply put, cannabis in drug form can be interpreted as a perfect symbol of the essential nature of Shiva. And if he can invent gambling, and condone hs followers' rampant use of cannabis, it's not a stretch to imagine that he would use cannabis himself.
Version 3:-
The question implies being high on marijuana is bad and why the hell a god does that.

The answer is simple. To nature, nothing is good or bad. Nothing is pure or impure. It is the society which invented good and bad and it's the hindsight which provides guilt.

Marijuana has many medicinal properties. When coming to its uses in meditation, its consumed by some Aghoris for getting a specific spiritual state which is far beyond our capacity of understanding. What we know about Marijuana is, it can be used for some temporary pleasures. What a yogi know is how to use it for a higher purpose. Obviously that's the reason the government doesn't arrest any Aghoris though they use them. these people are far beyond the restrictions of the law.  Lord Shiva is the Adiyogi [ The first of all yogis]

Version 4:-

Sadly, what people know about SHIVA is a sheer misunderstanding. Firstly, Shiva is not a person. Stop seeing Shiva as a HUMAN-GOD; that's the saddest interpretation of the Lord. Shiva is not a Human-God, he is described in the scriptures as Nirgun Brahmman. To elaborate further, the Shwetashwatara Upanishadh tells us that Shiva is:

1. Nirgun (In this state he is formless, and the whole universe and creation lies in the pervasiveness of Shiva);
2. Saguna (In the Saguna state, Shiva is the entire universe and his "ansh" is present in the tree, the insect, the animals, male, female and the whole of creation. In this state, though, all forms arise out of him, yet no form can describe him. He remains the causeless cause of all causes;
3. Nirgun–Sagun (In this state, through the intellect, knowledge, senses that are made ofgunas, we try to explain Shiva in terms of gunas and material forms yet our explanation cannot do justice to him as he is all that we explain and yet he is not all of that we explain.....he is beyond thought, intellect and sense perspection.)

In the Nirgun–Sagun state, Shiva is worshiped as the Shivalingam (contrary to the rumors spread by our Islamic brothers, that Shivalingam is the male genital. The fact is that Shivalingam has a deep and scientific theory and philosophy. The word Shivalingam derives from the Sanskrit roots Shiva (Lord) + Lingam (Mark/Chinha/Symbol). Hence, Shivalingam is the mark of the Supreme Godhead within Creation. Everything in the world arises out of a dome/a ball/ a pindi......be it a tree, which comes from a seed, which is round, or a child from a cell which is round—all heavenly bodies are round—the most minute of cells are round and our very planet is round. Everything, being round,  is a mark of the Lord/Shiva. Since Shiva can't be fully comprehended, we worship his mark, or we worship the whole Brahmman through a Shivalingam.

Beside its philosophical explanation of Shivalingam, there is also a deep scientific explanation, which explains the atomic and cellular theory. (I will throw light on this sometime later as that is a vast topic).

Many people take BHANG as the praśad of Shiva. This is a total misconception. When we offer the following to Shiva, it has a deeper philosophy and meaning.
1. BHANG- offering bhang to Shiva is symbolically saying this: "Oh Mahadev!! I offer all intoxication within me to you in the form of Bhang. I offer you my intoxication related to maya, women, fame and all bad habits. Just as u consumed the halahal duringsamudramanthan, be gracious enough to accept my intoxications and bless me with purity, knowledge and humanity".
NB: Shiva shastras are completely against Tamsic and Rajsic foods and drinks. A true Shiva devotee stays away from meat products since he cannot kill or be the cause of Pashupatinath's loved creatures. Shiva devotees also avoid alcohol, smoking, and onion and garlic. As for Nagas.....their Aghor shastra is totally misunderstood (but I will elaborate some other time). It also should be noted that the whole Universe is associated with Shiva, but Bhang is associated because of its highly efficient ayurvedic properties. Bhang is used in Ayurveda to treat epilepsy, skin issues, digestion, depression, mental issues, muscle pain, etc. But perhaps most importantly, Ayurveda prohibits overuse of bhang.
The Indian Subcontinent has abused bhang. Bhang contains no alcohol and is a safer drug than even cough mixtures which contain alcohol. Also, this Bhang pratha is followed primarily in the North. South Indians do not proclaim, "Bholay ka prasad and get high on it".
Too much of anything is bad, be it bhang, salt, sugar or excessive love-making.

2.DHATTOORA- offering dhattoora to Shiva is symbolically saying, "Oh Mahadev!! I offer all the poisons within me to you in the form of Dhattoora. You alone are Vishdhaari and I seek your grace to get rid of the poison of jealousy, fear, competition, foul language and unkind nature. Bestow upon me a pure and benevolent nature.

3.MILK- offering milk to Shiva is symbolically saying, "Oh Mahadev!! I offer milk so as to be graced by a Shweta character, mind and spirit just as milk."
4.YOGHURT- Yoghurt comes from milk but it can never be converted back into milk. Hence, offering yoghurt to Shiva is symbolically saying this: "Oh Mahadev!! I offer you yoghurt in the hope, though I come from samsara, that my devotion towards you may bring me to a spiritual level from where I may part from samsara and not be effected by its avidya."

5.GHEE-offering ghee to Shiva is symbolically saying, "Oh Mahadev!! I offer you ghee in the hope that you bestow me the prosperity of health, knowledge and wealth, and that I may use these for the benefit of others who are in need."

6.HONEY-  in offering honey to Shiva we are symbolically saying, "Oh Mahadev!! I offer you honey in the hope that you bestow me with a tongue that is as sweet as honey and that never pains a heart, but that brings peace and calmness to the ears and souls that hear me."

7.SUGAR-offering sugar to Shiva is symbolically saying, "Oh Mahadev!! I offer sugar to you in the hope that you make every cell of my body as crystal of sugar, sweet enough to dissolve all  bitterness."