I was told long ago that it is because I am the first born. If you are the first born in your family what's your default mode?
I was held up as an example to my "baby" sister and was told by my parents that I had to be a good role model for her. Whether that's why I am what I am today or not I cannot say. It's just what I was told. I can't shake serious. It is an automatic response to almost every situation. Serious. Are you serious too or is that a foreign land to you? It is a heavy burden and I don't recommend it to anyone.