The paint on all the drywall in the house peels off really easy. If you ever tape anything to the wall, the paint often comes right off on the tape. Not consistently enough to just strip the walls with tape, but often enough to be a problem. I don't know why this happens. Speculation is that they didn't prime before painting, painted on dusty walls, used really bad paint, or a combination.
Anyway, to get new paint to stick to the walls, you really need to remove the old paint first, since to paint directly on top of the old paint would just give you the same problem in a different color. We did this once with my first daughter's room. We manually scraped the old paint off the drywall, plastered the gouges and such, cleaned off the dust, primed, and repainted.
For future reference, like when we decide to paint the baby's room, I would like to know if there is an easier way to get the original paint off the drywall than just scraping it off dry. Is there any sort of stripping solvent or something that would work? Would steaming work, like with wallpaper? Scraping it was a lot of work. I'd prefer something else if possible. But the old paint definitely needs to be removed before repainting.