After the war against Gaea ended, Nico found himself at a loss of things to do. He stayed at Camp Half-Blood for a while but as always, all good things has to come to an end.
He grew further apart from his friends, even Will, whom had seemed like he would stay as Nico's friend for a long time, drifted away, back to his best friend Jake Mason.
Percy and Annabeth both stuck together, closer than ever after the events in Tartarus. Nico knew how they felt, he'd been there too, the only difference was that he'd been alone.
He had a feeling that his trip through Tartarus had made his darker thoughts resurface, the thoughts he hadn't thought for years.
He also figured that his darker demeanour was something that made him push people away from him more so than before.
Percy had picked up on his solitude faster than most, even faster than Jason – but that was to be expected, since he was never around anymore.
However, it was too late for Percy to change anything about the son of Hades, he was already too closed up for him to get him out of his shell.
He'd seemed sad enough when Nico finally said goodbye to them, probably for good – after telling the elder about his feelings, Nico didn't have anything holding him back.
Read the rest via the link in the description!