Why do people start off by saying “you’re gonna hate me” before saying something that will upset you?

[FallonSamuels]

[newboy78]

To get on a safe side by indirectly saying , 'please don't hate me for saying this'.

Saying those words puts a thought in another person mind to be ready for the bad news and getting themselves ready to try not to hate the other person.

[eagle1951]

trying to warm you before bad news

[AmandaYVR]

They feel it needs to be said, but they know there's a high risk of the person not receiving is happily.

You might equate it to 'tough love.' Not pleasant, but important to be said so that the person learns something from a situation, and how to properly assess it.

[Debbie14217]

bc they think its really bad news and they might know you well enough

[Shy_Steve]

Usually because they don't want to say "Sorry".