Hmmmm, I've attempted to explain my reasoning about 5 times; each time ended up with multiple paragraphs of an explanation which each time I didn't think quite got the right point across, so I'll summarize briefly:
I don't want to teach in an inner city.   Overall, I think there's just too much of a culture that doesn't value education.  I get my greatest enjoyment out of teaching kids who want to learn - to push them far beyond the required curriculum, and to spark as much interest in my subject matter as I can.    I do find it rewarding to save the occasional kid who is thinking about dropping out of high school - to get them to see the importance of education and at least stick it out long enough to earn a high school diploma; something they might be the first in their family to achieve.  But, for most of those successes have been with students whose goals are simply to enter the workforce, commonly working on their parents' farm with the long term goal of one day taking over the farm.  
My real reward is in knowing that I inspired kids to go much farther.   This year, of the graduating seniors who took physics with me, more than 1/3 of those are entering college majoring in something related - electrical engineering, mechanical engineering, even one student going to MIT for astronautical engineering.  After having fun discussions in class where I made it perfectly clear that it was only my opinion that manned space flight is a waste of resources that could be better spent on exploration with robots, etc. (Spirit, Opportunity, Cassini, Kepler, New Horizons, Curiosity, ...), BUT if he ever worked his way through the ranks at NASA and 30 years from now, they decided to attempt that teacher in space for PR thing and had a propulsion system that could send me to some place pretty cool (in the "neat" way, not the 4 degrees K sort of way), to remember me.