Why does Percy Jackson feel miserable once he gets his own cabin?

Percy feels miserable once he gets his own cabin because it makes him feel different.
When Percy first arrives in Camp Half-Blood, the training camp for demigods, he is assigned to the cabin of Hermes. This cabin houses Hermes's own children and serves as a temporary home for the demigods who are "undetermined." These are the boys and girls who know that they must have a godly parent but do not (yet) know who it is. Percy learns the sad truth that some of the demigods are never claimed and have to stay in the cabin of Hermes indefinitely.
Nonetheless, as Percy spends some time in the crowded cabin, he begins to feel as if he belongs. Before, he did not have any friends (with the exception of Grover) because he had frequently changed schools and was labeled with the diagnoses of dyslexia and ADHD. Now, he is surrounded by peers who have gone through and are going through the same challenges. For the first time in his life, he feels like something is going right.
After the game of capture the flag, Percy discovers that he seems to have extraordinary power when he is surrounded by water. Almost immediately afterwards, he is "claimed" by Poseidon. This is quite an event, as Poseidon is one of the "Big Three" (gods) and is not even supposed to have a child Percy's age. Everyone is forced to show respect for Percy, and he has to stay in Poseidon's cabin, where he is the sole occupant. While it is more comfortable and he does not have to worry about his things being stolen, he soon feels isolated from his peers. Again, he is labeled as different.