Well... sometimes i think the weed-induced depression is a phylosophical one, rather than a clinical one... like... weed makes one realize the uselessness, the emptiness of things... in last instance, everything we do is void, is illusion, as several religions states it... but most time we are not aware of it, so we live without thinking about it, we do things, we believe things, as if they were important. But then one smokes, and realizes nothing is actually important, and it makes one sad.
"For in much wisdom is much grief: and he that increaseth knowledge increaseth sorrow." Ecclesiastes 1:18.