I was recently nominated for a teaching award (though I didn't get it, my colleague, the much more deserving Ralph McNeal, did). As part of the application for it, I had to describe my teaching philosophy. My first thought was "oops, am I suppose to have one?" Eventually, I cobbled together some things that I thought sounded professorial, but I have been mulling over since why I actually put more into teaching that I need to.
I started off thinking in terms of personal calling. Well, I feel called to research and creating knowledge, but I'm not so sure about teaching. Eventually, though, I stumbled upon my answer, and be ready to be underwhelmed:
It's my job, so I should do it well.
At first this sounds trite. What about the larger purposes of education? What about enlightenment and passing on understanding to the next generation? For whatever reason, these don't motivate me.
Instead, I'm keenly aware that I'm paid to teach (among other things), and so I should do my best every semester and constantly seek improvement.
For me the more important question is not why do people teach, but how effectively do they do so.