Why is it so hard for me to cry?

so for some reason, it's always so difficult for me to cry. my mom's an alcoholic and when she "graduated" from rehab, we had to sit in a circle and tell her how much we loved her. it was a really emotional thing for everyone and i felt like an idiot being the only one not crying. not to mention i felt like an absolute jerk because my mom probably thought i didn't care, which isn't true. also, my two best friends got sent away to wilderness/rehab/therapeutic camp and it's just not really effecting me. it's not that i don't care, cause i do and they mean the world to me but i just can't cry! even when my friend got raped and had a miscarriage she was crying her eyes out at my house i tried my hardest to cry and show her that it upset me to see her like that but i still couldn't cry! what the hell is wrong with me? i do cry, but it's like on random days when i'm just frustrated. my brother said it's because i've been through a lot for such a young age. btw i'm 16