I would always ask, and if they say Ok wish "best of luck" and carry on. In the reverse situation, all you have to say (if help is not needed) "thanks, but I'm OK". Where does all this agression and not wanting to help come from? Maybe I'm just getting old(er!).
Motos do it too, usually a quick look and a nod, a thumbs up if OK and if not a lot stop.
There is nothing wrong with trying to be a decent human being, and rebuffs (as in the lady with puncture post above) only demean the person doing it. Take it in your stride, and carry on being happy.