Post HOO. Finally Percy had slain Gaea, but as he stood there with his sword logged into her abdomen she was still smiling. Gaea would eventually wake back up, but before she was put back into sleep she decided to do one last devastating blow. A blow that was directly meant for Percy.
She would take away the most important thing in his life before she went into a deep sleep.
With her gone Percy wasn't the same. He bacame distant with everyone. His heart grew cold and so he left. Having no reason to stay. Now a god, Percy left camp and went where the gods had no power.