...has a habit of throwing up when in the midst of extreme exercise. I've witnessed this several times over a period of years. I don't think that it's psychological at all but rather physiological. His body is simply rejecting what is an uncommon and unnatural occurrence- exertion. It's saying, "Hey, stop this nonsense and let's get back to basics here and walk- not RUN! Oh, so you won't stop. Well let me show you what I can do to those nice new running shoes..."
I find that I too have a visceral response to uncommon occurrences in my life. Change for me is hard. While small, incremental changes in life are commonplace, the bigger, wholesale changes that rock our foundation are rather uncommon. Thank God. Frankly, those changes often make me feel sick. Why? Because the outcome is unknown. Monte Hall smugly asks, "What's behind curtain #3"? Well, we won't know until the backstage flunky with the cigar pulls the rope and reveals either a lavish gift or a booby-prize. (tee hee- I said booby)