Why do I cry so easily?

It's one thing to cry when I'm sad, but to cry when I'm embarrassed? And this isn't "I ripped my pants" kind of embarrassment. I get embarrassed whenever I'm having a serious talk with an adult (it's intimidating). For example, my boss talked to me the other day about how he was sorry for the way he yelled at me earlier that evening, and I started crying. Then yesterday I went to the dermatologist for a checkup, and when the doctor was giving me instructions about how to clear up my face, I turned red and got that horrible tightness in my throat. I tried to push away the tears, but I cried anyway. I hate making people feel like they did something to upset me, because they never really do. Why do I respond like this? I feel like such a baby. I am almost eighteen and have been reacting this way forever, no matter what I do to stop it. (I even went to see a counselor, but that didn't help at all.)