I think this is a great point.
You can be in a terrific, healthy relationship with a wonderful person, but yet feel a sense of loss. Maybe because you are dissatisfied with your career or the direction your life is headed with, etc. The day has 24 hours and the only person you will spend all 24 with is yourself, inside your head. So if there is some aspect of your life that is lacking, for whatever reason, the other person's presence in your life won't make up for it because it can't. In the beginning you can maybe convince yourself that it does, but in the long run, the things you are unhappy about personally will still be there. You have to take care of you, bottom line, that doesn't mean to the exclusion of the needs of the other person or to some extremely selfish ends, but just that there are voids you cannot expect your lover to fill, and there are things which you will ultimately have to do for yourself.