I personally knew/know three people that started taking flying lessons and didn't complete
One, a college friend, because he died on a student solo flight where wind conditions were beyond his capability (one instructor I know said at the time that the FBO should never have let him take the airplane)
One, a former co-worker, well, I don't know why he stopped. He is certainly smart enough, coordinated enough, not afraid of flying, probably knows more about aircraft avionics and navigation systems than most people here on POA, ... Next time I see him I have to ask.
One, a former co-work, just couldn't get landings right. I think she had almost 100 hours (and at least a couple of instructors). She is certainly intelligent, good head on her shoulders, has a graduate degree in mechanical engineering, etc. She just couldn't put it together.