I think it's a fine line. I told my friend the first time I tried and I'll not forget the fear in his eyes, I felt so bad for him.
We still talk etc but he doesn't - and I wouldn't expect him to - say "Alright mate, still planning on killing yourself?"
The way I see it the reasons for this are:
1. If I wasn't thinking about it then drawing attention to it might lead me back on that path (even though I am still on it, he doesn't know that)
2. It belittles the subject matter if people mention it every time you see them
3. Similar to the above, he doesn't want that to define who I am - that "guy who attempted/wants to commit suicide".
So he treats me normally, as before. I can understand the vast majority of people do not want to commit suicide and therefore do not understand where I am coming from. That's fair enough, more health to them I say.
Ultimately I think it depends on how you see that other person and what you want from them. They don't know what you want at all times. And as suicide is such an alien concept to most people then they are scared of it. If I knew someone who, for example, told me that God was talking to them then I wouldn't ring them up in the middle of the day and say "Hi buddy, just checking - did God leave any messages for me?" as it's just something I cannot understand.