Conditioning
Conditioning is the way repeated experiences train you to respond automatically, so a cue triggers a reaction without a decision. People-pleasing is largely conditioned: certain looks and tones pull a reflex you never chose.
When something happens enough times with the same result, your brain stops deliberating and starts predicting. A cue arrives, the response fires, no thought required. This is efficient, and most of what you do all day runs on it. It also means patterns laid down early can run for decades on autopilot.
For people-pleasing, the conditioning often happened young. If a parent's approval rose when you were agreeable and fell when you had needs, you learned, again and again, that pleasing brought safety and asserting brought cost. The lesson sank below words. Now a frown can pull a yes out of you before you notice you had a choice.
Because it was learned, it can be relearned. New experiences, where you pause, hold a small boundary, and survive it, start to lay down a different track. The old reflex may still show up. With repetition, it stops being the only thing that fires.