Back when I had my “what on earth is the point of seminary” phase (you folks may remember the thread), I had a lot of friends who were a bit flummoxed about it but let it go, a few who wholeheartedly agreed, and one friend who bugged me about it and did her best to make sure I at least applied to sem- she told me which one I should go to, why, what I’d get out of sem, and that she was going to make sure that I went.
Long story short, I’m applying and I’m happy I am.
Basically, if someone’d reached twelfth grade and decided she doesn’t want sem, I’d ask why, but it’s none of my business- only if it’d be a close friend asking (like that one was). I think part of the reason why people tried to nudge me about it was that they knew the reasons why I didn’t want to go and thought I was wrong- in the case of that friend, not so much wrong as delusional :). If no reason is given, then it’s just decent manners to drop it. None of anyone else’s business why- it could be financial problems or a medical problem keeping someone from going, or even just her not wanting to, which is also valid.
TL;DR: I think it’s fine (and caring, in its way) to ask why someone’s not going to sem- it’s NOT fine to keep bugging about it unless you know EVERYTHING behind the scenes and you know the non-sem-goer doesn’t mind.