Do you "like" me?
Most of the time when I like something it's because:
It's a mate and I want to support them
I like the sentiment
I like the content
I want to be one of the masses
A popular person/star likes it so it must be worth liking
And when I don't like something it's because:
I don't like it
Nobody else likes it so obviously not worth liking
Nobody cares so I don't care
I don't care
Information overload - I'm onto the next thing
We all feel better when someone likes our post. We're accepted! We're liked! Social media is social validation. Digital back-patting. Digital back-scratching. I'm sure there's another back related analogy in there somewhere. Oh yeah, digital back-stabbing.
I've been working in the wonderful world of content for well over ten years now. I can tell you that having more likes means you're more likely to get likes. Having more likes means people take notice, means being seen. Sadly, few take notice of the unliked.
In terms of behaviour, the "like" is more of an afterthought. You don't view a piece of content and think "I'm enjoying this so much I'm going to like it". It just happens.
Famous/massively followed people can tweet/post the phone book and receive thousands of likes instantly. That's the fan's like, the strength in numbers, all for one. Kanye tweeted i.e. automatic like.
Which brings me to dislike. Social media has focused on approval, rather than disapproval. Disapproving takes a lot more effort, a lot more work than approval. A nod of approval is essentially a like.