According to Aristotle's Rhetoric, there are seven causes of human motivation:
We're going to add one more that he missed: for Science. He was probably just being polite. The guy credited with setting down the scientific method as inductions made from known certainties probably just didn't want to toot his own horn. Plus, Science really fits into many of the other categories, also. We enjoy doing Science. It creates pleasure. Therefore it is an Appetite. But also, maybe our personal goal is to achieve or understand truths, and therefore Science becomes part of Reason. Or maybe, for some high schoolers, it is a Compulsion because they gotta take this freaking Earth Science class to graduate. But even so, the fact that the motivation "for Science" fits into so many of the categories explains why it needs its own. So here you go. Here's your excuse. When you're telling the cops, the jury, the Nobel committee, the National Academy of Motion Picture Arts and Sciences why you did it? Tell them the truth. You did it for science!