I think... I think he has issues stemming from fear of rejection and abandonment and a rather crushing fear of loneliness, so it's probably partly from his family's indifference to his pain, partly from being separated from Kurt that it's more evident now than when they are together and happy. I like to think it becomes less up and down as he gets older, and he can find an even keel to settle into once they are married and start a family. Though I'm sure triggers like the first time Christopher gets hurt, or Kurt is late coming home without notice could start a burgeoning panic attack that could turn into an episode.