What's wrong with me?

This is something I always find myself thinking about...what's wrong with me? I mean...I am always depressed, always on the verge of crying, leaving for months and just wandering off..but why? I mean..most people would think my life is good. I have clean nice clothes, food, a job, plenty of people who like me. I shouldn't be feeling the way I do...but I do. Like...something is missing in my life but I can't piece together exactly what it is. I guess the only thing I can do is take in each day and see where it goes.