When someone habitually says yes to something, either without reflecting on whether they want to do it, or because they dislike or fear the consequences of saying no. Often women who have been brought up to be "nice polite girls" suffer from yes syndrome.
If you want some help, just ask Lisa. She never says no to helping anyone, because she's got yes syndrome.
I wanted to call in sick for work because of my cold, but then I didn't. I must be suffering from yes syndrome towards work.