Anas رضي الله عنه narrates that Rasulullah صلي الله عليه و سلم said:
He who meets his Muslim brother with what the latter loves in order to make him happy, Allah shall make him happy on the day of resurrection.
[the rewards for good deeds- page 492- Hadith number 1534]
SubhanAllah!!! Just by doing an action that would earn your Muslim brother/sisters happiness, Allah will make you happy on the day of Qiyaamah.
One action can give you such a big reward!!!!