Selfishness in Christians is akin to a kid not sharing a French fry when his dad owns the whole McDonald's.
"You have not lived today until you have done something for someone who can never repay you." - John Bunyan
Whoever is kind to the poor lends to the Lord, and he will reward them for what they have done. - Proverbs 19:17
Give, and it will be given to you. A good measure, pressed down, shaken together and running over - Luke 6:38