Pre-Crisis continuity: He felt he had outgrown the role and that Robin belonged with Batman but since Dick was pretty much out of Gotham and hanging around the Titans, he decided to pass the mantle to Jason Todd. It was a peaceful transition and Batman was okay with it.
Post-Crisis: He didn't quit, he was fired.
He quit being Nightwing because Bruce died and there was no Batman in Gotham. Villains and criminals had a field day terrorizing the city since Batman wasn't around. Realizing Gotham needed it's protector, Dick took over the mantle of Batman and gave up on being Nightwing.
John and Mary Grayson
Dick Grayson was the original Robin .
While it is never explicitly stated, there are several clues pointing to his last name being "Grayson".
Umm, no. He will be in his own book returning as Nightwing come the relaunch.
Richard John "Dick" Grayson became Nightwing after a falling out with Bruce Wayne.
Dick Grayson was created in 1940.
Pretty sure it was Robinimproved:-Yup, Dick Grayson is robin. :)
John and Mary Grayson
Mitchell Grayson.
Dick Grayson does not actually have any superpowers, he was the original Robin and latter became Nightwing and then Batman.
Dick Grayson
Female
Yes
Robin (Dick Grayson) .
Robin's full name is Dick Grayson. He was the original Robin and later became Nightwing.
Dick Grayson
It doesn't seem likely.