I came up with this question when I was talking with my friend the other day. I wanted to ask why he didn't want to try something.
At that time I said 'Why aren't you trying?' instead of 'Why don't you try?'. Because I thought 'Why don't you/we/I/etc' is used to suggest somehing, not to ask reasons.
Did I say correctly? I think the nuance is slightly different from what I intended. It sounds more like I was blaming him.
What should I say when I really want to ask the reason why others don't do something?

Right now I am getting curious about one more thing. Is 'Why don't she/he/they' possible? It doesn't seem to make sense to suggest something to a person who is not here now.

I know these questions could be silly to native speakers, but I want the answer badly.
Please help me.

Thank you.
Hi Aedilis, and welcome to English Forums.

If you want to know why he doesn't do something (usually something that you think he ought to do), you can say "Why DON'T you [send Christmas cards][visit your ailing mother][use deoderant]?" It's the same in writing as "Why don't you X?" but the emphasis would be on "don't" and you would use it for something that could/should/would be done habitually.

"Trying something" you do only once, so asking "Why DON'T you try it?" sounds like he has frequent opportunities to try, all of which he always turns down.

Simply say "Why don't you want to try it?"
I think you're correct in your analysis of the situation. We often don't like to be pinned down with questions about our motives. Hence the great popularity of the reply, "None of your damned business!"

Therefore we approach with caution. It's always a sort of cat-and-mouse game - even among the best of friends. Personally, I don't like to play that game - either as the cat or as the mouse.

If someone asks for a suggestion, I try to be as direct and honest as possible. "If I were in your situation, I would etc." OR "Have you tried doing XYZ?"

You're right. "Why don't you XYZ?" would be taken as a suggestion. If you're working with a friend, trying to fix something - change a flat tire - and the guy is doing something really stupid, and having trouble with it; you may or may not want to offend him. I'd probably ask him if he could do me a favor and XYZ. That way, I'm not actually correcting him.

Sometimes you really don't know if the person has even considered what you're about to suggest. That's why I prefer "Have you tried XYZ" to "Why don't you try XYZ." It's less confrontational.

But when push comes to shove, as they say - when you know he doesn't want to do it and you really think he should - and it's important to both of you - and you really don't know his reason - "Why don't you try - - - " Why aren't you trying - - - " Why haven't you tried - - - " are all okay, but they may not make it clear that this is a sort of "intervention" on your part. That is, you've made a decision to stick your nose in his business, as a friend, for his own good. ASK the reason. "What is the reason you don't want to XYZ?" He might appreciate the direct approach more than if you keep beating around the bush. (You may both discover that the reason can easily be dispensed with.)

Re your other question, "Why doesn't Joe want to join the club?" Of course when Joe is not present it can't be taken as a suggestion to Joe, and would be understood as asking Bob about Joe's reasons.
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Thank you Grammar Geek. Thank you Avangi.
I'm more than pleased to hear(read?) your explanations. This forum and its members' kind help are priceless for a person like me.
I appreciate it.

Thank you again.
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i learn a lot of thing here. thank you