Why would a girl start crying because you yelled at her?

I pulled a prank on my girlfriend, i had her phone and i texted flirty messages to my friend,
i did record it but it went wrong.
I had pretended like she had sent the messages and i got on her phone to look at them
when she saw the messages she was lost.. i started yelling at her (i wasn't serious)
but i was really yelling.

She tried to yell back but she just broke down crying & i didn't mean for it to go that far so i just cut off the camera and told her i wasn't serious blah blah
she stopped crying eventually

i was holding her/ kissing her/ doing anything to make her feel better..
she was mad at me, she made me walk 3 blocks down just to buy her a little carton of ice cream so she would feel better

i didn't think she would cry..

Why would a girl start crying because you yelled at her?
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