That rush of focus and borderline obsession at the beginning of relationships is generally referred to as "New Relationship Energy" or "NRE" around here. If you do a search it is talked about in various situations around here - I'm not sure how many threads are actually tagged NRE, but it is mentioned pretty much anywhere that there is discussion about new relationships.
I think it is part of a natural progression, but it's important not to let yourself go to far with it. For example, when I begin new relationships, I have to make sure I'm not going to overboard and ignoring the relationship I have with my husband. I have to make sure I don't move too fast or break and boundaries that have been set up or do something that makes my husband feel like he isn't important anymore. In general, I think people also tend to make promises that they aren't prepared to keep when in the throes of NRE. It's easy to tell a person that you'll love only them forever when you are so focused on them that nothing else matters, but as soon as that initial frenzy wears off you start noticing other people and that promise may prove to be premature. It's easy to overlook issues in the relationship or differences between you and the other person at first, but as time goes on those differences and issues can seem more important.
I enjoy the rush of feeling so incredibly into another person, but I don't trust any of the imaginings of the future that may enter my mind or any long-term promises/goals discussed.