The scariest, closest to suicide moment I ever experienced was when I was shopping at a mall. It wasn't very busy, and I was feeling very... manic might be the word. Head down, headphones in, speed walking between errands. I had to keep moving, no matter what. I noticed the sort-of cut-away in the floor, so you could see from the top floor down to the ground. It was only 3 stories, but I had a really strong, sudden impulse to vault over the railing and see what it would do to me. Maybe I could land on my head, that might work. It was something I had never really considered before, it wasn't a very efficient way to do things, but it just gripped me very suddenly and all of a sudden it was a huge crisis.
Being suicidal, generally, is death by a thousand cuts. You have a predisposition, and all the stresses in life push you in that direction (as you've said). But the jumping off the ledge moment is crucial; personally, I have strategies for daily temptations like putting away knives in the kitchen. It's the spur of something unexpected, the sudden, strong, irrational impulse that really scares me.
You're right that most people who want to die don't want to die all the time, and suicide decisions are impulsive to the extent that that they're made when the suicidal person wants to die (which is only some of the time) and not when they don't.
However, I think even when the decision is "impulsive", it's a decision to execute a plan whose details have been worked about over a long period of time. Usually suicidal feelings aren't acted upon the first time they surface. I'd imagine that it's actually very difficult to kill oneself, and it takes a lot of preparation to do it right, and a lot of determination to will oneself into following through with it.
I think "I love my friends and if I kill myself now I'll never get to see them again" is a valid reason to keep oneself alive. But shaming and pressuring suicidal people, making them feel bad (as if they don't feel bad enough already) for the impact their decision will have on others is totally out of order. Everybody has the right to choose to end their life and they shouldn't be shamed for it. At the same time, that choice does not exist in a vacuum, and it's worth looking at the external pressures that influenced the person's decision, and external entities can absolutely be blamed (and held responsible) for a person's suicide.
(I have dealt with some suicidal feelings before, but who hasn't? I don't want to come across as authoritative on this: I'm speculating based on my own experiences and those of other people with whom I've talked about this stuff. I'm completely open to correction and to hearing other people's perspectives and I appreciate your comment. I have no time though for the kind of victim-blaming that some people are doing in this thread.)