Give for the inner joy of giving, and not for external rewards.
Humans forget to give gratitude; it’s only natural. Even you, at some point in your life, must have forgotten to express your sincere thanks when someone helped you out. If you expect gratitude from all those whom you help, you are likely to be disappointed at least some of the time. This disappointment will lead to dissatisfaction and unhappiness.
Giving simply for the inner joy of giving will make you much happier than any reward ever could.