Because Batman will always need a Robin. He gets sloppy in the field without one.

( more violent. more reckless. jason wasn't there after he died, but he's heard stories. and bruce has had tim or damian ever since. he won't dig into the bruce wouldn't mess; jason can think of a lot of shit he thought bruce wouldn't do, but did anyway. )

You're just a brat now--what, twelve? Thirteen? You think you'll want to play sidekick to B for forever? And when you move on--how do you think B's going to take it?