I think to an extent, you do have to be happy with yourself before you can engage in a serious relationship.
In the past, my relationships have gone downhill simply because of the way I'm feeling about myself. I've been with my current boyfriend now for over 3 months, and the first two months were absolutely wonderful. Then - due to certain situations I was in - I became stressed. I had quite a big self image problem and don't think very highly of myself, so when I'm stressed or upset I start feeling worse about myself. Because of this, it was putting strain on my relationship. He wasn't doing anything wrong - he hadn't changed whatsoever. It was me who had changed.
During the past 2 weeks, however, things have got a lot better and I genuinely feel happier in myself. Through this, I can see that it's having a positive effect on my relationship. :] If that makes any sense?