Why are some guys so secretive/set a barrier with what they share with a girl?

Anonymous
I have a couple of guy friends I love hanging out with. We see each other every week over dinner or drinks. We've known each other for a year plus. The thing is, unlike my other close friends, girls and gays, we tell each other stuff easily from family stuff to their dates encounters etc. I enjoy my guy friends company but they seem to keep personal sharing separated when we hang out. I'm not a nosy person. I always believe if a person wants to share something personal, they would in their own time, at their own terms. These guys share minimally about their family and almost zero about their girlfriend or ex girlfriends. I am interested in one of them romantically, hence, the curiosity, but even with the other guy, who is his best friend, when I asked him about his ex (whom I met already and we're all friends now), he asked me why I'm curious before he answer me but he shares more compared to the one I actually like. Are guys generally secretive or are they just selective with whom they share and I happen to be someone they choose to be mysterious with? I hardly ever ask personal questions but I am myself pretty transparent about my life with them since I've been friends with them awhile and we hang out weekly. Just curious.
Why are some guys so secretive/set a barrier with what they share with a girl?
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