THe group I hung with in highschool was definitely the druggie group. NOt one did it "because it was illegal". Most of it was for the experience. Yes, its a mood alterer, doesnt mean you rely on it to alter your mood all the time or that your mood is sad/depressed without it (unless you are talking addiction which is NOT the same). I never did hardcore as I never wanted to. I only smoked pot as I felt nice and relaxed on it, but I didnt really drink when I was younger as I didnt like the way it made me feel.
Heck, a really good yoga or meditation can alter my mood for sure, or a glass of wine in the evening, or a bath, or sex, etc. Marijuana does have health benefits as well.