WHY DO GUYS pull away or withdraw when they get close to a woman?!!!!!?

I'm asking this as it says it in "Men are from Mars, women are from Venus". It mentions that when a man reaches a level of intimacy with a woman meaning they get very close mentally, he feels the need to withdraw and be on his own, its therefore up to the woman to understand this and relax i.e. don't chase or call him or even worry because he'll be back and when he does, he'll want to continue getting close to the woman. My bf does this often enough. When we spend a lot of time together, he'll call and see me less for a few days and then will want to really see me and be extra loving/sweet/attentive. It drives me crazy b/c I always worry that something is wrong. Guys, do you get this?!