the truth
Anteros' biggest issue is he knows that he was born to be Eros' playmate, which to his mind makes him feel like Aphrodite and Ares didn't actually want another child. In fact, Anteros bases his entire existence on Eros and his whims, so if Eros ever decided he was unhappy with Anteros or didn't like him Anteros would be utterly bereft.
Beyond that, he has all manner of feelings related to being gay and how that's affected his life. Though his sister seems to accept him he knows that their parents have certain expectations regarding his duty to marry and give them grandkids that he'll never be able to fulfill, and that there's a good chance they'd disown him if he ever formally came out to them. By that same token he also worries that Ares would never accept that one of his kids wasn't a total bad ass. Sure, there was a time when Anteros was the avenger of unrequited love and made certain people paid for dicking around others, but these days his only effective attack involves using his words.