Probably contentment, that horrible trap that comes with relationships. You feel fulfilled emotionally because you have a partner to take care of and be taken care of, so a little switch goes off in your head that you don't need to preen and attract a mate, so you don't take such good care of yourself. I've seen it happen to loads of people, I've even dealt with it. The only way I can deal with it is asking my girlfriend to tell me off for not working out or to tell me she doesn't like it when I eat cheap foods. It makes relationships a little bit more stressful, but she never means it, and I think you need to be kept off balance a little to stay on top of life.