Why do my brothers put me down?

Not that I'm the perfect sibling, but I always seem to find that my brothers put me down behind my back and sometimes blatantly to my face. I always encourage my brothers and tell them how proud I am of them, but they always find something negative to say about. My oldest brother claims that I've been babied and that I'm lazy (I'm really just trying to figure my life out), and my second brother is always telling me that college is a waste of time and there's no guarantee that I'll get a job (which I do understand, but I'm about to graduate and my future is already worrying me). I don't understand why they do this, I really just mind my business not bothering them. Any suggestions on how to ignore them and keep pushing forward.