Many reasons. They usually boil down to her trying to get her way in a certain matter or her being guilty of something that's eating away at her.
Many men get immediately disarmed once they see a woman crying and their instinct is to immediately comfort her and give her what she wants.
That is exactly what she is trying to manipulate you into.
Makes sense. I was too hypnotized at the time to see how a lot of her crying was fake. Now that I am educated and see everything for what it was, I realize how she used to cry within secs to get what she wanted. Whether it be to comfort her, feel sorry for me, forgive her for something that any normal man would have left for, or to get information.
One night we were eating dinner and she started crying, telling me I should have never broken up with her, how I broke her heart, my dumb azz apologized like a chump, when the reason I did break up with her was justified and any healthy man with respect would have left long before this happened.
Then 10 mins later, she got up to use the ladies room, only to come back within a min with tears in her eyes, asking me how I found out about the damaging lies that caused the break up. Now I see how she was trying to make me feel guilty first before asking for what she really wanted...To find out how I found out about her BS. She didn't give a crap about my feelings, only hers. I didn't tell her how I found out, but I wont deny her crying worked a lot of the time. Looking back, I feel so much shame for being so blind to the obvious. When I analyze the 2 years I spent with her, it really makes me feel stupid for letting a women treat me the way she did. This girl could have one an Oscar for being able to cry on demand. Sad that I still love her after all she did.