As a skeptic, I often find myself questioning my own intuitions. It's a very healthy thing to do, in my opinion. Physicist Richard Feynman once mused, "The first principle is that you must not fool yourself -- and you are the easiest person to fool." Our intuitions are subject to all sorts of personal biases that have been ingrained in us by genetics and experience, mostly unbeknownst to us, and therefore don't necessarily have any truth behind them. Instead, they do little but serve our desire to pursue our own interests.
As a matter of fact, humans are self-interested. That's just the way it is. And, yes, to a large extent, most human behavior can be descriptively summed up as either a product or byproduct of adaptive evolution. I fully admit that this includes most moral behavior as well. People generally act according to the desires of others because it is within their interest to do so. The point of this post is not to deny this most basic fact, but instead it is to question whether bowing to self-interest is reasonably valid.
Acting in one's self-interest seems intuitively reasonable, but I believe that this is nothing more a product of a very strong bias. There is nothing reasonable about considering your own interests whilst not considering the interests of others. If this is correct, it at least puts a selfless person on equal rational footing with a selfish person. But, I think it can be demonstrated that being selfless is in fact more reasonable than being selfish.
A selfless person views the world in a more impartial manner. Where the selfish person only considers his own interests, the selfless person considers the whole scope of interests when choosing a course of action. He takes more variables into account, and sees himself as but one conscious being in an entire society of beings, all having interests of their own. He understands that, just as he values his own interests, others do the same, and that his own interests are no more intrinsically important than another's.
This, to me, is a much more reasonable position to take. It is the difference between blindly bowing to the external and biological forces that forged you, and actually applying critical thought to how you ought to act. It's the difference between being little more than a dog tied to a cart and being a freethinking individual.