Why do guys get uncomfortable when girls cry?

I guess I never really understood why men get nervous or uncomfortable when women cry. I've only had one serious relationship and I never cried in front of him. I almost did once and he immediately apologized and was sweet and caring etc. He seemed so nervous that I was upset that one time that I tried my best not to laugh.

This conversation came up the other day with some girls and there were many different answers, but I wanted to hear some responses from guys as well. Any reasons?
Why do guys get uncomfortable when girls cry?
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