Because they are looking for something. You won't find true happiness in all that 'stuff' of the world, so you look for it elsewhere, in drugs, alcohol, sex, whatever. We seem to have to learn where happiness is NOT, before we can truly learn where it IS!! I think it lies in learning the truth about life in the first place. Why are we here? What is life about? Who am I really? What is my purpose here? I think those are some of the most important questions in life. Finding those answers can take a lifetime, and much life experience. Good luck to you lostsoul! I hope you can find some peace of mind, some true love, and real lasting happiness.
It is there, and it is the greatest treasure we can have.