I think the whole question is more about why we listen to the music that we listen to, period. With Dream Theater, they're so diverse that they have a song for every mood. Sometimes I'm in the mood for something heavy, or something with chunky riffs, they have songs that are good for that.
Sometimes, I'm in the mood for something with a lot of intensity and atmosphere, something that helps my creative spark, or makes me feel like I'm watching an action movie, they have songs that are good for that.
Sometimes, I want something soothing and mellow... Well you get the idea. I can honestly say that DT has a little bit of everything for me.
But when I'm in a general Dream Theater mood (which is often), I listen to it just for the sheer love of music, like, I can feel the passion and the joy the members felt when writing and recording it, or playing it live, it really comes through in the music, and knowing the band as well as I do now, it's very uplifting to hear.