It’s heavily hinted in comics that Damian’s blood has magical properties because of the lazurite pit, so what if Damian had powers but the only way for him to use them is if he bleeds.
People find out about this and obviously they want his blood and Damian feels obligated to give it to them. Tim and Bruce want it for experiments they need to find out what are the limits of his blood’s capabilities? Dick wants it because it holds the key to solving a case he’s been on for a while. Jason needs it because it can heal a chronically ill girl that lives in his apartment block. Word spreads and the Teen Titans want it. The JL wants it. Young Justice want it. Justice league dark wants it. Soon everyone in the hero community want it.
So Damian bleeds and he bleeds and he bleeds and he bleeds. Until it gets to the point where he can’t stand up without severe vertigo and he’s passing out in school. His hair is falling out, he’s turned sickly pale and he’s got all of the other nasty side effects of blood loss.
Damian can’t do this anymore. If he keeps on doing it he’s going to literally bleed himself to death. But he can’t say no because he’s devoted himself to fighting evil and his blood is aiding others in doing that. He’s 15 and faced with the moral dilemma stop giving out your blood to everyone who asks and face the wrath of everyone around you or continue to give out his blood ultimately killing him.
That’s it, that’s the idea.