I have a friend that has been married for around 5 years now. We have been friends since college and has known the husband since around that time. They have always looked good together and it came as no surprise that they eventually got married. Now blessed with two Kids, they are one of the few couple I know who I'd personally vouch will be for life.
I may not know much about the husband and what goes on in his thoughts, but my friend had more or less settled on the idea of him for a very long time before they got married. She had choose him even before they started to take their commitments seriously. Such early stage loyalty has always left me quite amused.
The sheer amount of doubts and hesitation that creeps in when am romantically involved with someone at the early stages is pretty significant. It does gradually fade over time but I don't think it ever truly disappears. For instance I think I have found love, but am conscious of the fact that life is unpredictable.
One of the biggest reasons why early stage decision making might be critical is because it spurs useful action. If your heart is set on someone, then you can go all in and enjoy the wealth of experience. Of course for perpetual fencists, that experience is never fully realized, or at least stretched out in small doses over a long period of time.