[Lewis Thomson (OP)]

Emily has been puzzled over this psychological question for a while,

"Why do humans build neurological pathways to reinforce behaviour that often initiates an adverse response after the first encounter? (e.g. when first time drug users experience a toxic response to the drug what builds continued use? The body recognizes the toxicity of the substance inducing an adverse response and why the brain seeks consumption thereafter?) The surface level response indicating dopamine activation seems contrary to an appropriate evolutionary response. Additionally the supposed purpose of plant toxicity is defensive, so why do animal brains build 'reward' from it?"

Can you help her in the comments below?...

[alancalverd]

Either the pleasure of intoxication exceeds the pain of the toxic response, or people persist because they can see someone else enjoying it when they have become habituated.
My guitar teacher told me on my second lesson that if it doesn't hurt at first, you aren't doing it right. Then he played a test piece and explained that, like ballet, it is worthwhile learning (painful) classical technique if you want to play jazz, but very difficult to do it the other way around. I've never been tempted by hard drugs or aerobatics, but I guess the same applies. 

[evan_au]

Why do humans build...

Maybe it is the plants that build chemicals (eg caffeine, opioids  and cannabinoids) which mimic animal neurotransmitters and bind to animal receptors to manipulate their behaviour?
- Caffeine has been shown to improve bee memories, so they can find their way back to other members of the same species of plant to fertilize the flowers.
- Some plants might benefit from plants eating and spreading their seeds, providing they don't crunch them up and digest them. So feeling pleasant about swallowing the seeds might encourage animals to swallow them. But the overdose from chewing them might encourage the animals to let them pass through undamaged...

Human agriculture means that we have a year-round supply of concentrated drug, when in nature it only occurs in dilute form for a limited season.

If people become accustomed to a regular supply of these imitation neurotransmitter, their nervous system adapts to them, and reduces production of the natural neurotransmitter. This means that addicts suffer withdrawal symptoms, and feel the need to go back for increasing doses of them.

Of course, with modern chemistry, we have ways to produce a whole range of chemicals that play with our nervous system in various novel ways. Some people just seek novelty...
https://en.wikipedia.org/wiki/Humphry_Davy#Pneumatic_Institution