It's good that you're taking his secret to the grave, so to speak. His disappearance is more than him being him and it sounds like it's necessary to pinpoint which disorder creates disappearance and development of a new love interest?
Does this person use/misuse substances? That can create disappearing acts when combined with a variety of disorders. Maybe the excitement of travel creates a sense of identity? The normal feeling of elation can be euphoric for certain disorders. The crash feels ten times worse than exhaustion after an adrenaline rush. Will he return? Maybe after he stops chasing after himself.