Selfish vs. Social
During the eternity that I spent in college, the most important thing I learned is that all life is selfish. Selfishness can be "beautiful" in it's own way. Consider the lilies (cf. Matthew 6:24-34), all competing to have the most attractive flower, so the bee will pollinate the seed, so the lily will live on in the next generation. Every plant and animal in an ecosystem uses what's available, selfishly, to survive and reproduce.
But like storing nuts for the winter, there is no "OFF" switch to tell the squirrel it has more than it could possibly ever need, so it deoesn't need to be selfish any more. Consider a lion, with it's "Red in Tooth and Claw" behaviors it cannot survive without. An ecosystem is "beautiful" until you or yours are being hunted, then all of a sudden it seems downright cruel.
We share the same DNA as those plants and animals, so it makes sense that we inherited those selfish behaviors. Scientists actually have a hard time understanding the difference between animals and humans, but the authors of the Bible have drilled down deeply into their souls...
For I do not understand my own actions. For I do not do what I want, but I do the very thing I hate. Now if I do what I do not want, I agree with the law, that it is good. So now it is no longer I who do it, but sin that dwells within me. For I know that nothing good dwells in me, that is, in my flesh. For I have the desire to do what is right, but not the ability to carry it out. For I do not do the good I want, but the evil I do not want is what I keep on doing. Now if I do what I do not want, it is no longer I who do it, but sin that dwells within me. So I find it to be a law that when I want to do right, evil lies close at hand. For I delight in the law of God, in my inner being, but I see in my members another law waging war against the law of my mind and making me captive to the law of sin that dwells in my members.
Most people would not say that animals sin, but if animals were humans those selfish acts would certainly be called sinful. We commonly restrain and even kill animals for the good of everyone, but we generally don't blame them for doing what their genes tell them to. What the Bible calls sin is part of our DNA, and that is nothing to be proud about. Sin is bad for everybody, and everybody includes "me too."
What would happen if you grew up alone, without even a wolf for a mother? You certainly wouldn't play well with others. If everyone was raised that way, selfishness would seem completely natural. If everyone were that openly selfish it would be anarchy, and history has never known a society built on anarchy that works for long. "Do what thou wilt" is the devil's law, but the opposite is "love thy neighbor" (Mark 12:31b).
If you like tech, think of this example. Imagine starting from scratch, without language, and having never even seen a bow and arrow. How difficult would it be to develop everything we have now? Though we often don't like to admit it, we have others to thank for the life we live. And it's hard to imagine anything that has been built without some form of cooperation.
But progress hasn't been steady. Many times in earth's history, selfish people have taken more than they deserve. They have destroyed what cooperation has built. So to be fair, people have always needed laws.
Then consider how many of us have been influenced by Christianity through the centuries. Jesus wants us to build each other up, not tear each other down. Many Christians have been "good" for others because God has been good to them, so we have all been blessed with that goodness. God is opposed to the proud and greedy who give themselves more honor and resources than they deserve (James 4:6, 2 Peter 2:3). Doesn't it seem possible that Christianity has influenced all cultures for good? As a country, everyone knows that the United States has been successful, but many do not thank Christianity and God's law for that success:
Beware lest you say in your heart, "My power and the might of my hand have gotten me this wealth." You shall remember the LORD your God, for it is he who gives you power to get wealth.
Like the economics of Capitalism, selfishness makes very good sense of how things work. But gathering far more nuts than we need becomes obviously unjust to everyone (except those who call those cruelly selfish desires good and moral). Jesus didn't say it was wrong to take care of ourselves (how else would we be able to take care of others?), but He warned us about seeking things for ourselves before seeking for God (e.g. Matthew 6:33).
Jesus summarized the Bible's message with the life He lived (Ephesians 1:10). He wants us to be givers, not takers. He wants us to cooperate with one another. But He also warned us not to give and cooperate when it is good for ourselves and take and cheat when no one is looking (John 3:19). Taking more than we need is natural. Loving yourself and your family more than others is also natural. But Jesus asks us to do more than just what comes natural.