I hadn't cried since I was thirteen time until a certain film came along called Sacco e Vanzetti. It still makes me cry. I didn't cry after that until I fell in love.
I think it has something to do with how close you are to the people in your life; how much you care about them. The more you have to lose in your own life, the more the losses of others affect you. The happier you are, the sadder you can be. Your emotions polarize when you have something to care about. You still have a spectrum, but the extremes aren't as extreme as those who are more emotionally invested.
It can also be the opposite. Sometimes you are so full of feeling or have experienced such extremes that almost everything else seems like a shadow of true sadness.
Complex stuff, emotions.