Seth Godin:

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.

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.