Hi, Anonymous --
Great question! The answer is not too complicated, and it's really the same for men and women. The reason is "comfort level." Relationships go through stages (as you know). The initial stage is infatuation and "hunt" -- both parties want to please each ther and do everything they can to show their loyalty, commitment, and liking of the other person. They put the other person first, and they do what they can to show that person that he/she is doing the right thing by being with them.
The problem with this stage is people tend to believe that it will remain like that forever. Usually it won't last bast 6 months, most frequently it begins to burn out after 3 or 4 months if you see each other everyday. Why? Because that level of effort is very hard to maintain. It usually means you put the rest of your friends second, you put your whole like "second" frequently, to making the relationship work. That's just exhausting. Often you will do things you don't really like, just to impress or make your partner feel good.
The second stage is pull-back and assess. In this mode, you stop being infatuated, and start looking at your "partner" from the perspective of "long term." That just means you begin to be yourself again! You start going out with friends, you begin to be critical of the faults you once ignored in the infatuation stage, and you start trying to integrate your partner into your own world -- but on your terms. This is often viewed as "pulling back" because you are no longer solely focused on making your partner happy, or always worrying about what they want. This can lead to fighting and anger if the partner is still in the infatuation stage and feels cheated, or if the partner does not like when you STOP making them the center of your world.
The final stage is "maintainence mode" -- in this mode, you compromise. In pull-back, you wanted to get back to your friends, and you wanted to pick-up the pieces you left behind when you just went crazy over your partner. Now, in maintainence mode, you strike a balance. This is can cause serious problems too because it requires that both parties be mature enough to realize that they cannot be the sole focus, yet have to give up some of their own life in order to make the partnership work. That level of maturity can kill many relationships.