I just found a really cool blog called Sliding vs Deciding, by social psychologist and relationship expert, Scott Stanley. Among the interesting topics, “Coasting and Drive-by Opportunities,” “Oxytocin: I Feel Your Pain,” and “What Drives Sacrificing for a Partner.” One theme that emerges throughout: men and women related differently, and part of the differences are physiologically driven.
From his post called, “Well Its Men: Does he flip for her?”

I think commitment for the average man is a bit more like a light switch that gets flipped on (or not) at some point with a particular women when it comes to commitment. It’s flipped or switched on once he becomes clear that she’s who he wants to be with in the future. Until it’s flipped, he may be in love and he may be great to be around, but he’s not crossed over to where he’ll give regularly for that partner without resenting it. I think the average women crosses over to giving more fully sooner in how the average relationship develops. So, if we have the average women and the average man in a relationship together, early on, I’m betting she’s going to move more quickly to fully to sacrificing than him.

Think about that. There’s no great problem if this is true except where the guy never catches up. And that’s why books like “He’s Just Not That Into You” are bestsellers, because it too often never does catch up. If commitment is more like a switch being flipped for the average male, women are at greater risk for over-giving in romantic relationships until he flips—for her. Based on this theory, I’ve often suggested to women that they be careful not to give too much until they can find the switch and see if it is working. This advice is just as good for men, by the way, in relationships where they are the ones to give too much until the commitment is becoming clear.