This is my theory, at least.

People dislike themselves because of their thoughts, but other people like them for their actions.

Thoughts are hard to control; especially if you have issues with mental health and self esteem, your thoughts can go to very bad places, and make you feel like you’re awful. Actions require more effort, and people can take the things you do and say- the things you consciously choose to put out there-and they think of you as good and likeable because of those actions.

Basically, you’re not as unlikable as you think you are. In fact, you’re very likeable, and judging yourself because of errant thoughts is unfair to the wonderful person you are.