Gender disappointment.

My girlfriend and I just found out that we're going to have a boy. I could tell that she was a little upset at first but I expected that. Once it started to set in she completely broke down in tears. I can't wrap my head around why she is so distraught over it being a boy rather than a girl. To be honest it really upset me that rather then be excited over all of things we will do with him, she is in tears over what she might have been able to do of it was a girl. Right now she's so upset she won't talk about it and when she calms down she won't talk about it. I'm lost right now.