I have a theory that Trump not only boycotted the dinners for a few years because he's so fragile about things said about him, but mainly because he is self conscious about this: He has no idea how to make or interpret a joke. That's why he does accordion hands and repeats his quips so very often. It worked in the old movies people laughed at back in the black-and-white days! He doesn't know how dull or stupid he sounds because he very much does not understand humour. He only (fake) laughs when safe people around him laugh. And he is acutely aware of that.