How come there a no "happy" DT songs?

Dream Theater seems to be in the depression zone. There are not too many songs that DT plays that really make you smile. I guess surrounded is kind of uplifting but there are really no DT party songs. You guys have stuck with prog metal but I know that you at least are into bands such as oysterhead or system of a down that kind of play whatever comes out. Have you guys ever written any songs that were just too happy for DT and shelved them? DT is clearly capable of putting out an awesome commercially oriented album. How come you don't do it?