Why do l cry when lm angry but not when lm sad? ?

I am 17 years old and ever since I was little, l have cried whenever I get really mad. This is pretty much always when my parents yell at me in my room when we are alone, and not when it is in public. Whether it be over grades or, like tonight, over my driving, I start to tear up. Its usually when most people would start yelling, but Im not allowed to yell back at my parents. Its not like I cry at anyother time, though. Even when my granpa died, I cried for about a minute when I found out, and a tear or two when they closed the casket. I didnt cry at the wake, the funeral, or even when they put him in the ground. I firmly believe that this has to do with my parents and the fact that they hit me as a kid (they didnt beat me, just hit me when I did something bad). It probably also has to do with the fact that I run away from fights and challenges. I most likely is the reason I dont like my parents and my brother. I dont HATE them, thy just annoy me and I dont like being around them. I really think there are some psychological issues that I have to deal with, but I just want some feedback or something. And believe me, im not trying to be a *****, because I hate it when people always complain about nothing, but I kind of think this is serious. Thanks fo any feedback. Much appreciated.