It's not natural or enjoyable for animals to become intoxicated.
While I wouldn't get my dog drunk or stoned, I just had to jump in and dispute this statement. My cats are the biggest nip-heads in the world! They'll do ANYTHING to get stupid on catnip (and they get funked up, too! swatting at things that aren't there, rolling around like the floor is the best thing in the world...I have to hide the catnip in an airtight container in the cupboard to keep them from it). Monkeys eat hallucinagenic plants to get high, birds and elephants eat fermented fruit to get drunk, moose eat mushrooms and trip their brains out.
We're not so different in some ways. Animals have even sometimes shown humans where in nature the best drugs are!