1 The post was emotionally provoking
People like posts when they are emotionally provoking in a positive way. If the post is emotionally provoking enough, the person might share it as well if it is the kind of post that could be shared.
2 The post matches their views
If the post matches a person's views then the person is very likely to like it.
3 Good relationship with the one who posted it
People generally don't feel comfortable liking the posts of the ones they don't like. Even if a person writes a good post, it still might not be liked by the ones who don't like the one who made it.
4 To socialize
People sometimes like the posts of others just to socialize and connect with them. A person might like your post, if he doesn't disagree with it, just to get closer to you.
5 To compliment others
The motive behind the like can also be complementing others. If a person makes a certain achievement then his close friends might like his post just to compliment him.
6 They like the person
If someone likes a certain person or has a crush on them then the former is very likely to like the posts of the latter. See signs that someone likes you.
7 The post is amusing
People like and share jokes online when they find them amusing or when they help them express their opinion in a funny way.
8 Matches their interests
People are more likely to like posts and pages that match their own interests. A person will usually feel annoyed when he sees an unrelated image. See why Facebook likes are important.
9 To express approval
People usually like posts to express their approval of what is being said or to express approval of the person who wrote the post.