Why is it we always want whats not good for us?

Whether it be food or in relationships we always seem to crave the things most unhealthy for us. Now the really bad thing about this all is, We can see when other people are making the wrong choices but can never spot it in ourselves or just don't really want to. I seem to be making the wrong choices alot lately and can't seem to stop myself. Its like all self control flies out the window and I'm watching a carwreck in slow motion. I wanna stop myself but can't really seem too.