More often than not, the selfish person is insecure, fearful and filled with doubt. The selfishness springs from his belief that this is his only good idea, his last dollar, his one and only chance to avoid failure. "I need this, not you," he says, because he truly believes he's got nothing else going on, no other chance, no hope.
The irony, of course, is that selflessness (not selfishness, its opposite) is precisely the posture that leads to more success. The person with the confidence to support others and to share is repaid by getting more in return than his selfish counterpart.
The connection economy multiplies the value of what is contributed to it. It's based on abundance, not scarcity, and those that opt out, fall behind.
Sharing your money, your ideas, your insights, your confidence... all of these things return to you. Perhaps not in the way you expected, and certainly not with a guarantee, but again and again the miser falls behind.
(This is part of what Sasha's generosity day experiment is about.)