Why do some people do selfless things?

"We do not exist for ourselves", wrote Thomas Merton. At the end of life we will not be judged by how many diplomas we have received, how much money we’ve accrued, or much of anything else. We’ll be judged by:
I was hungry and you gave me something to eat, Hungry not only for bread — but hungry for an embrace and for love;
I was naked and you clothed me, Naked not only for garments — but naked for dignity, respect and acceptance;
I was homeless and you took me in, Homeless not only for four walls with a roof — but homeless because of rejection and being dismissed.
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There's a void that exists in many of us. It nags at us. It’s to be purposeful - to make a difference. I’m thoroughly convinced, true happiness and genuine significance doesn’t come from finding yourself, but in losing yourself - by investing in someone else. I like what A W Tozer wrote: "The purpose of good works isn't to change us or save us; rather, it's the demonstration of the change within us."