One key theme is alienation. Seymour seems to be alienated from the world around him. He can't really connect with the other guests at the hotel.
Another theme is the loss of innocence. The little girl, Sybil, represents a kind of innocence, and Seymour's interaction with her shows his complex relationship with that innocence. He's been through a lot, like in the war perhaps, and has lost his own innocence in a way.