People share things that can help them support their important points of view. If a person has a certain belief and finds a post that supports that belief, there is a very high chance that he will share that post.
People can get frustrated as a result of daily experiences. One of the tools people use to release that frustration is to share posts that allow them to a vent. A post that makes fun of a politician is a good example of posts that are used to release frustration.
People want others to see them in a certain way. One of the tools people use to express their identities is by posting certain things on social networks. A person who wants to appear brave can post a picture of himself attempting a dangerous move (See why people buy branded clothes).
People Share a post when the emotional arousal they experience after seeing a post exceeds a certain threshold. If a post makes a person emotional, most likely it's going to be shared.
People can share certain posts to get approval from friends. A person who shares a joke with friends usually expects his friends to laugh and think that he is funny.
A person with a beard is very likely to share a post that explains why bears are a great thing. Some posts give people a self esteem boost and that's why they are shared by those people.
People share posts that best describe them. When a post describes a person's core beliefs and helps him express his personality, he usually shares it.
People can share posts to put down people they dislike. If a person doesn't like what a celebrity did for example he can share jokes about him just because this makes him feel a bit better (See also why some people hate you).
Humans are social beings and they can hardly survive without connecting to others. One of the main motives behind sharing is connecting with others (See why do humans join groups).
Some people share certain posts in order to show that they are smart or even superior. A comedy video that shows some of the funny mistakes random people make is a good example.
Posts are sometimes shared to promote a certain Idea. A person who belongs to a certain political party might share some posts with the goal of motivating others to believing in his party.
Studies have shown that people who write about their problems usually feel better. People who also make fun of the bad situations they face are more likely to feel better.