Mainly because I'm a really, really non-spontaneous person. I do give fairly large amounts to charity, close to 10% of my monthly income right now, but to be honest, I like my routine and I have a hard time departing for it, and taking a semester off to go build schools in Africa or something would be very much outside of my routine, would set me back by a year in my program, etc, etc. My best hope is to plan, non-spontaneously, to do things like that. For example, once I'm graduated as a nurse, I'll feel much more secure moving somewhere for, say, a year to do volunteer work... I'll be able to settle in, get to know people, establish a routine, and in general set up the conditions that work for me to enjoy myself.