High on chemicals your own body produces or high on chemicals you introduce into your body to simulate the effects your own bodies chemicals produce?

Starts with therapeutic reasons pain either physical or emotional

I, for one, have never liked the feeling of being either drunk, or high. But, I know people who like to use pot to help them relax.

Escape ..if temporary…

"Catnip (Nepeta cataria) is a perennial herb that is a member of the mint family. It can grow to be up to three feet high! The chemical compound in the plant that attracts and affects cats is called nepetalactone. It is found in the leaves and stems."

It serves the same evolutionary purpose for the mind that random gene mutation serves for our physiology. It allows us to escape the set patterns of our thinking long enough to experience random or unfamiliar patterns, which we bring back to the real world, throw them against the wall, and see which ones stick. Most don’t, but the few that do contribute to our competitive fitness in the form of creativity.

Probably for a pleasurable experience.

Its not just humans.