Jason at A Small Change is hosting this month's giving carnival on the theme, "What motivates people to give?" I suspect that a true inventory of reasons would plumb every vice and every virtue, every quirk, of which human nature is capable.
Eve's gift of the apple, or for that matter Jehovah's gift of the Tree of Knowledge, the gift of free will, grace, inspiration, hope in dark times, and the shedding of redemptive blood are all gifts. Poetry is a gift from the gifted, so is a knack or craft passed on from mentor to apprentice. Giving is a way for the Emperor to display his largess, another realm for the CEO to conquer. A tomb of lapis lazuli to house his earthly remains may be the Bishop's gift to his parishioners. Dying to save another in a fire, or on a battle field are gifts, as is buying a bottle of Thunderbird where a nickel goes to feed the homeless, as part of the Red campaign. Mother's milk is a gift. Giving to cheat death, giving to cheat the tax collector, giving to spite an ungrateful heir; giving because you don't know how to say no when asked. Giving as a dog might be tossed a bone. Giving as a sign of disdain, the coin tossed in contempt. Giving as status competition, like potlatch. Giving as social obligation, family tradition. Giving as ethical or religious imperative. Giving as justice. Giving as atonement. Giving as reparations. Giving as an investment with an eye to political return, or graft. Giving as social investment with an eye to others giving back to you. Giving as a social investment for a real rate of return. Giving as a social investment for the good others. Giving as the expression of personal identity. Giving out of superstitious dread.
I once worked with a man who was suddenly wealthy, as his business unexpectedly went big and went public. Asked why he gave his answer was, "Because if I don't, God may get pissed off and take it all back." (It is ok he used bad language; he was worth $100 million.) We all give for our own mixed reasons, as we marry or have children for motives many and mixed.