My ex's DS is like that, and has been through out his life, he is now 7 and continues to wail for any reason you can imagine. I understand he is a very sensitive kid but I also believe that his parents have not helped his sensitive nature by validating all the fears he has, ie:
Child has a go at a climbing frame, has a little slip and starts crying, and instead of dad asking him to dust himself off and try again, his dad shush him kindly as if he were a little baby and tells him he shouldn't have taken him there as it is not suitable for children, such a dangerous park, then tells him that the climbing frame is not suitable for children, he shouldn't get near to the artifact ever and tells him he is not allowed to play in any other areas than those for toddlers even when he is seven.
If he has a little incident, they make a huge fuss about it. For example a couple of months ago dad called me to say that after an accident during a walk, he had to carry the child for 2 miles because he had hurt his leg and "there was blood everywhere". I saw the child next day and the "wound" that caused such drama was a a little graze the size of a 5p coin.
But the worst comes from the dreadful descriptions that the parents go into when they want him to behave in certain way... ie child starts running in the park and dad screams " Oh please J don't run, you are going to fall and scratch your knees, a bicycle will run you over and split your head in two!" Needless to say the poor child is terrified of life itself... as soon as he fell he started crying in horror expecting his life to end. I don't blame him... I would too.
I'm sure you are NOT doing anything like this to your children but if you do, please don't. Believe me, sometimes telling a child to pull themselves together is kinder in the long term.