I don't know if I match the criteria, but you might want to consider that a naturally lower arousal threshold is just your lot in life. I myself never was of the "stiff from a gentle breeze," variety, even at my hormonal peak in the early 20s, and I just assumed it was natural. I was an athlete, lived a pretty healthy lifestyle, and was constantly around attractice women and having lots of sex. Despite all that, I never seemed to be as conventionally 'horny' as most of friends.
Also, worrying about it may create some mental blocks and stress that could interfere with the process. I doubt anything is wrong with you physically, so it seems likely that it's more of an emotional/psycholigcal issue. This doesn't mean you're screwed up or out of whack in some way, it could be something as simple as needing a change of environment or meeting a person that's appealing to you on a different level than you're used to.
Whatever the reason, I wouldn't dweel on it too deeply if I were you unless it's making you serously unhappy.