Cooperation

Sometimes very bad things happen and people act together. Cooperation is a lot like self-control in an interpersonal way. If self-control is about "punishing" your current self for the sake of your future self, then cooperation is about "punishing" your own interest for the sake of other people's interests—which might become yours sometime in the future.
In a place where everybody cooperates, people share interests and goals; in a place where nobody cooperates, people fight against each other pursuing individual goals and interests. Everywhere in the world there are people struggling for common interests against selfishness and there are people taking advantage of some altruist fellows. But cooperation isn't just something that can be imposed over people; that's why very bad things need to happen before people start acting together.
The bad thing is that bad things are not the same for everybody, although there are some natural disasters, for instance, with which most of us would agree to say that "a very bad thing has happened." Still, there are bad things that affect most of us and not everybody calls them "bad." A very bad thing, then, needs not only to affect a lot of people, but also to emerge suddenly. Most of humanity's worst problems are everything but sudden, so not everybody cares.
Selfishness, for some, is a very bad thing, but it happens. Altruism, on the other hand, is a very good thing, specially for selfish individuals, and it also happens.
Cooperation is the art of finding common interests even among selfish individuals—an art for which altruists can do a lot. That's how cooperation isn't something that needs to be imposed; it is more like self-control—an intention that needs to be put into practice so its results can become evident.
As long as there be humans in the world—that is, for us, always—bad things will keep on happening. We are, nevertheless, capable of making a decision as to if it is really necessary for a sudden, very bad thing to happen before we find common interests, or if we can set as our common interest to cooperate with others. Even if it somehow feels like a punishment now, our future selves will feel rewarded.