Equivalence is the first, most simple condition of cooperation. Everyone of us is different, and the idea of being equal seems right but in reality it is not. If I am short, I shouldn’t eat the same meal as people that are half meter taller than me. I wouldn’t need that much food and they would need more than mine.
A bigger meal for a tall individual is equivalent to a standard meal for a shorter individual. Kids may eat more sweets than adults, but should eat a lot less wine. Diabetics may want the same sweets as their kids, but alas, they know it wouldn't be a good choice. And so on: individuals and their differences should be considered for equivalent benefits, not equal ones, in order to live equivalent lives.
Cooperation is different than altruism
Cooperation is different from altruism, solidarity and donation where we see a “recipient” that simply “gets” in a one-way action. It is easy to say that if you follow only your personal benefit, you are in the world of egoism. Sometimes, instead, you are only focused on giving and you forget about yourself. Cooperation is an equilibrium of the two, a situation in which you're selfish while being altruist -and feel happy about it.
As described in the proportions page, a simple selfish or altruistic attitude is linear, it goes directly to the point, not being aware of the ecosystem (people, environment, space). Cooperative behavior is complex: it is way more than the simple idea of giving or getting.
Benefits and relations
You may enjoy food, a new car, and also a good friendship. There are two main balances to be respected: an equivalence of benefits, that happens when you get equivalent goods, services or valuables as the other participants, and an equivalence of relations, in which you are as averagely happy as the other participants are.
Cooperation happens therefore when both equivalences are met, the first one is good, it is called a win-win, but it is not enough. Cooperation is not a mere goods and services exchange, but a relational exchange too.