Where did I ever say it had to be any reason from any comic? Beast Boy could spitball it, Starfire could suggest it, he could see it on the side of a cereal box for all I care. They had a perfect opportunity this week, with Donna. It could have been a name she called him when they were young to tease him, or taunt him. But they passed that up. At least for now.
I'm just saying it's lazy writing if they don't introduce the idea somewhere before he adopts it. We should see the moment he first encounters the idea of Nightwing, however it happens. Even if it isn't necessarily the Nightwing name, but rather the costume. The suit, the name, something inspires Dick to choose the identity he goes with. They've gone to such lengths to show us how Robin hasn't worked, they need to give Dick some reason to believe the new persona will work better.
Example in point: Daredevil never names himself Daredevil in the Netflix series. It's a name given to him by the media. First as "The Devil of Hell's Kitchen", and then later as Daredevil proper. But the series does show us Matt's thinking in choosing to specifically adopt the devil iconography when he has Melvin make him the suit at the end of S1 (Father Lantom's conversation about the devil being the hound on the heels of sinners, chasing them back to the side of the angels).
That's all I'm asking for. Give me a reason why he chooses that identity. If not the name, then the suit. And "just because he's a superhero" may be true, but it's an absolutely lame justification in a world that has gone so far out of it's way to ground it's characters.