The moment you have a self at all, there is a possiblity of putting yourself first - wanting to be the centre - wanting to be God, in fact.
That was the sin of Satan: and that was the sin he taught the human race. Some people think the fall of man had something to do with sex, but that is a mistake. (The story in the Book of Genesis rather suggests that some corruption in our sexual nature followed the fall and was its result, not its cause.)
What Satan put into the heads of our remote ancestors was the idea that they could 'be like gods' - could set up on their own as if they had created themselves - be their own masters - invent some sort of happiness for themselves outside God, apart from God.
And out of that hopeless attempt has come nearly all that we call human history - money, poverty, ambition, war, prostitution, classes, empires, slavery - the long terrible story of man trying to find something other than God which will make him happy.
~ from Mere Christianity, C.S. Lewis