One who is very interested in social reform and helping others. The term is often used pejoratively to suggest that such a person is naïve and their efforts futile. I know you're a do-gooder, but how many people do you really think you'll be able to help on such a small scale?
do-gooder
(ˈdugʊdɚ)
n. a person who is always trying to help others. (Often derogatory.) I don’t consider myself a do-gooder, but I try to help people.