It wasn't that he didn't love her.
It wasn't that he was betrayed.
It wasn't that he was depressed.
It was that he couldn't live with the guilt.
There wasn't an explanation to his guilt. It wasn't even his fault. Technically, it wasn't the Gods fault either. They were forbidden to meddle with their children's life and therefore could not do anything about it themselves.
The Greek and Roman campers could have done something to stop it from happening yet chose not to. The issue had been raised at a meeting once and there had been a vote but it was dismissed after there were not enough in favour of helping. The hero of the two camps had tried to argue, insisting that it was a problem that needed fixing, even if there were few that would do something about it.
They still wouldn't listen.
He fought until some campers dragged him out of the room and calmed him down. After that, he refused to come to any of the meetings. Quite a lot of the campers started to lose their respect of him. Some began to think that he was overreacting and thought he should get over his 'petty issue.'
So he decided to do something about it.
He went to the one person who had agreed with him during the meeting. A boy who was a few years younger than him but just as mature as himself. The two met late at night in the forest beside the camp.
The older boy tried to convince him to come with him and after many minutes of arguing, he finally agreed with one condition. That he could ask another person to know, and that he could come and go as he pleased.
The older boy made them both swear on the River Styx that if the boy and his friend did not agree with his terms or did not want to stay with him after they left, then the could return but could not speak a word of where they had been and what they had been doing.
A few days later, they decided to disobey their leaders.
A few days later, they left the camps.
A few days later, they disappeared.