Yep, nothing that plays with your brain chemistry is good for you. Over a period of maybe a million years, nature crafted the human brain with a hell of a lot of natural safeguards, one of which was against excess seratonin or other 'pleasure causing' compounds. Probably this was because being too happy was bad for our survival. Seeing what crawls out of nightclubs in the UK at 4am in the mornings, I can quite understand.
THC (edit) has been linked with (mild!) psychosis for many years now, a link denied by pro-cannabis campaigners, when they can be bothered to deny anything.