I'm not sure how people suddenly got confused into thinking that wasn't a hypothetical. :/

Anyways, the reason people regret the stuff they don't go to is because the conclusion is unknown. It could have been good (which is what people cling onto/count on when they have said regret) but it could also have been very bad. I find as long as you keep a neutral/realistic outlook on the past, rather than an optimistic it's easy to not regret skipping ______. Cause you remember you're reasoning, and how likely it really was to turn out not so well. 

Also, often when people regret such things they're looking at it in a sense as an outsider. They are no longer in the situation, dealing with the pressures, the 'right here, right now' sensation and experience. It's easy to say "You should go to _______" when you're not in the situation anymore.