This could well be a case of trying to add “too much value,” and here’s the problem: the quality of the idea may go up 5% with my suggestions, but your commitment to its execution may go down 50%. It is no longer your idea; as your manager, I have now made it my idea.
Or, as Rob puts it:
Taking someone else’s idea and increasing the quality by 5% occurs at the price of a 50% decrease in their commitment to execution.
It’s utterly obvious, and I think we already know this stuff deep down, but having it packaged up in a single sentence like that is always great useful.