One guy liked something, and then he wants to share it with his friends. The thing, apparently, makes him so happy and he wants to share the happiness.
Aforementioned are two scenarios that are quite typical. In between people, sharing is common. And happiness is meant to be shared. Photos are happy little containers of happiness, and this wide web-connected world means a lot of sharing is possible.
Come up with a great photo, get it online, and get more friends.