Nature is cold, hard, and ruthless, and only the most aggressive - and selfish - survive and pass along their genes. So would suggest one standing school of thought about the nature of the world, a philosophy that plays itself out in the writings of authors like Ayn Rand, who elevated selfishness to high ideal, the most powerful force of creativity and industry possessed by humankind.
As it turns out, this isn't merely an oversimplification of the natural order of things - it's probably mostly wrong. A team from Michigan State University used a logic model to demonstrate that exhibiting only selfish traits would have spelled the end of the human race a long time ago, and that cooperation and mutual benefit are, in fact, core to our success.
This makes sense, when you look at nature for even a moment - families of animals (and insects, and other organisms) take care of one another, communities defend themselves, and even entire species join forces with one another in displays of symbiotic mutualism that work to the advantage of all those involved. Communication and memory mean that short-term boons gained by trickery and selfishness tend to poison the community to the detriment of all - and disrupt potentially positive cooperation.
Examples abound in the natural world - everything from the remarkable teamwork of ants and their helper species (such as aphids) to the co-evolution of flowers and pollinators such as bees (which have their own fantastically cooperative hive structures) to human interactions with livestock and pets.
Something is given, something is taken, and (generally) everyone's better off for it. (Ask a factory-farmed pig or chicken about this, and you may get a more negative spin on what "cooperation" means in this context, of course.)
And in a family, too, there's a dual challenge at hand: to teach cooperation among family members (including convincing siblings to stop whaling on one another long enough to enjoy each other's company or help one another with chores), and also to teach good citizenship - which is to say putting aside selfish impulses in order to better the community as a whole.
Weave strong enough webs of community and it pays off in more ways than a deserved sense of self-satisfaction - people who live in so-called "blue zones" of longevity are seen as greatly benefiting from high levels of social engagement and rich relationships with nearby friends and family.