They say, that humankind has a natural, inexplicable, karmic gift: what goes around, comes around. Actually, that is only half the circle. If you want the full circle to exist, there is an accompanying responsibility: what comes around, must go around. In simple words, that means everyone must be at the giving end when they can give it, if they wish to be at the receiving end when they need it.
Human society is huge. We number somewhere in the neighbourhood of seven billion people, every single one different from the other. We live in organized cooperative society (or at least, most of us do), where each one must contribute, so that the collective good is served. So why is altruism necessary? Why would one want to selflessly help someone else, without any expectation of reward?
It makes immediate sense when one says that each person must contribute; each one must work and earn a living, and the combined contributions of each person help sustain the community as a whole. Earlier, we had the barter system so the reward for contribution was direct; now we have money, so it's a little more complicated, but it's still essentially the same in principle. But it wouldn't make immediate sense to someone extremely practical, that going a little bit out of your way to help out others without any expectation of reward is just as important. You don't get a clear answer to the question: "What am I going to get out of it?"
Anyone who's interested in why our ancestors developed a sense of altruism and why we still have it today may refer to the relevant chapter in The God Delusion, by Richard Dawkins. The book is about atheism, but Dawkins is an evolutionary biologist, and he offers an explanation from that point of view. Altruism is essentially an investment, due to the extreme unpredictability of life. I cannot be sure of getting food everyday, but if I live in a group where everyone tries to get food, and shares equally whatever is obtained, surely my chances of starving at the end of the day are significantly lesser, than if I were to live alone. I share my excess today, in the hope that tomorrow someone will share their excess with me, if mine depletes. Individuals who possess this sense are more likely to survive, more likely to reproduce, and more likely to pass on this trait to their offspring, which is why we have it today, so many generations after it developed in primitive humans.
There can still be an objection raised at this point. Alright, so someone does you a favour. Why should you not return the favour to that person, the one who did it to you? That person deserves it more than anyone else, why must one be altruistic in general? And isn't such an investment a dangerous one to make at all? What if you do something for someone who will never be capable of returning the favour back to you?
There is nothing to prevent us from being good to people who do us a good turn. In fact, it's a very nice thing to do. It makes perfect logical sense. And as for someone who can't return the favour back to you, how do you know that there isn't a third person, who can do you good, but who is in need of something which the second person can give? How do you know that there isn't a chain of such people? The whole concept of altruism is based on this, that it works in a circle, and that such a circle is possible, never mind its actual realization in life.
The movie Pay It Forward was based on this idea. You may not always be able to return a favour that someone does you. Someone else may not be able to return you a favour that you do them. So why not just go on helping people anyway? Someday you will receive help too, because you are part of the circle. And the more people you help, and the more you encourage this concept, the bigger the circle can be.
The next observation is, how do we know that there is such a circle at all? There are hundreds of people in the world who do not receive the help they need, at the most critical points in their lifetime. Isn't the circle supposed to include everyone, if it exists? It's true that there are people who don't receive help. There are people who are harmed for no fault of theirs, often even deliberately harmed by someone else for that someone's personal benefit. But that makes it all the more urgent for people to understand and adopt the concept of altruism. It's true that you need to be able to support yourself at a minimum level, before you can support others. But doesn't it fit in, that others can help you reach that minimum level, from where you can take off and then begin the paying forward process yourself?
Most of all, it often doesn't take too much effort to go out of your way to help someone else. Helping out can be for things as simple as helping your neighbour with heavy bags, or giving someone a lift. That kind of thing is not just manners; in a way, it's altruism. If everyone gives it, everyone receives it too. And if you receive it, you should think about giving it too.
