[Twosugars]

From personal experience I can say that suicidal person think they're doing everybody (and themselves) a favour by "catching a bus" (killing themselves).
I can only talk about suicide caused by depression. Severe depression feels like living in unbearable anguish, feeling almost physical pain of living in hopeless despair. And like with any unbearable pain, sooner or later, you'd anything to make it stop.
Sadly, treatment options are far from perfect and being in treatment on nhs, for example, can actually make you worse iykwim. So at the end of the day is all about personal strength and will to survive, both of which can and do get exhausted sooner or later.