Why do some men seem to loose interest after the "chase" when courting women?

Ive noticed that in some of my past relationships some guys are a lot less attentive after they've secured you as their girlfriend. The flirting prior to the actual, "Will you go out with me?" is a lot more exciting and attentive then when you finally are actually in the relationship. Is this just because men want their prize and once they have it theres no need to continue the up keep?