why be compassionate??

the question i ask myself now is why be compassionate. why DO we feel the need or desire to help other people when they are in need? 'people are selfish and are motivated by their own self interests.' that is a quote by Forrest C. Greenslade in his book about how to motivate people. but if we are motivated by our own self interests, is it in our own interest to help someone who is in need because we notice it? i am having trouble seeing how this two issues work. maybe there is no such thing as compassion, maybe we have a word for it but in actuality it is never really practiced? but when i say that there are so many things that come mind, examples that prove the exact opposite of what i have just said. a very recent example that i can think of is that of Dr. Paul Farmer. He is a doctor who works in Haiti to provide health care for the sick and poor. he doesnt do it for the tax break or the press coverage, he does it because he believes it is the right thing to do. you can read all about it in his book ''Mountains Beyond Mountains.''