I liked Twain when I was a teenager and read his book about Jacob or his other short stories. I dont know how I feel about him now though! But I agree with him. I think people are - not completely but to a large degree maybe - calculating beings. People who help others obviously do this because it's something they like. It's something that makes them feel good about themselves. Like when I was a undergrad, me and my friends ran a student group for teaching orphans and helping them with their studies. At that time I didnt think about my motives or explore deeply how it made me feel, but now that I look back I realize that it made me feel like I had spent my time on something worthy and valuable and had become a better person! Helping others helped me as well. So what I want to say is that it's not necessarily a bad thing. Even if people help others because of self-interest, it doesn't mean others wont benefit from their help. Every human interaction is mutual and reciprocal. Altruism is not different. But it's reciprocity is spiritual and emotional.

I remember when I was younger, our teachers in school always told us that good people who help others go to heaven, it's their reward. But it made me upset because I thought it reduced what I did to trade. Doing something cause I would be rewarded after death? I felt degraded. So in a perverse way I refused to help others for some time. Like I wouldn't give my seat in the bus to the old lady or refused to give money to a beggar in the street. It took me few years to discard that stupid view and reconcile.

Pardon my rambling :D