The answer to this would vary from individual to individual.
For instance, if someone talks less and like silence, for her/him anyone who talks too much would be highly unlikable. However, these same talkative people would be BFFs to those who also talk a lot.
For people who are honest, someone who lies would be unlikable. However, if someone who lies herself/himself, the liars wouldn’t be a big deal to them.
I can draw many such examples. So, "what makes someone unlikable" -- the answer would vary for different people.
But there are some general core features that make anyone obnoxiously unlikable. Here are some of those features:
· Someone who forgets names: Imagine you’re talking to someone and she/he forgets your name. Some people genuinely have a hard time remembering others' name. And then there are those who think it's cool (to act) to not remember the names of other people. In any case, you will feel bad. And there's a good chance that you might end up disliking that person, at least in the short term.
· Someone who question others' intellect: "You are wrong" "you don’t know anything" -- if someone tells you such things, it would not only instigate your ego to prove her/him wrong, you would also grow to dislike that person with the 'how-dare-he/she to question my intellect' mindset.
· Someone who just talks about self: Unless you're in love with that person and genuinely wants to know more about her/him, this is a bad trait. You want people to ask you questions. You want others to know more about you. If they don’t, and if they are all me-me, there's a good chance that you might end up disliking them.
· Someone who lies: Generally, the majority of the people dislike others who lie. Even if those lies are minor and don’t even matter, you would, in your heart, usually have a feeling of dislike towards that person
· Someone who talks about others behind their back: Unless that person is your best friend, you will dislike people with this trait. More so when you realize that such people also wouldn’t mind talking bad about you to others behind your back.
· Someone who's incompatible with you: "Girls shouldn’t wear short dresses" "I hope India loses today" "arrange marriage is the best" "depression is all in people's head". If the other person doesn’t match your ideas and opinions, and you’re exposed to her/his ideas and opinions for a relatively long period, you would dislike them.
· Someone who forces self to be a leader: You likely have a few people like this in your circle. You do not appreciate them for pushing themselves to be at the top and pulling others down. After all, even when you're not a natural leader, you like the idea of being in the center and leading the pack. So if someone always acts as a leader, by being a bully, pushy or negative, you won’t like that person.
(Courtesy: The Atlantic)
These are some common features or traits of unlikable people. Of course, there are many more.
I would assume you're asking this because you want to improve self and become more likeable. A simple approach in this case is self-reflection. Audit yourself and find out the kind of people you dislike. Create a list. And then make efforts to NOT be like this "kind of people".
Hope this answer helps!