Some people carry the desire to kill themselves sometimes for years, even planning it for that long. There is no perfect answer since there is so many variables to each situation. I do believe there is an issue with some meds, it has been mentioned but I know of cases where people that one would never consider suicide suddenly go off the rail and do it. Those that are serious about suicide rarely exhibit signs.
I absolutely believe this to be true. Had a friend kill himself in the front yard of his parents house, because of a simple argument. No rhyme or reason to his decision - easy going guy that was popular in school. His brother said that he had already put the shotgun in his car before the argument started. My uncle shot himself in the spare bedroom while my aunt was on the phone with my grandmother. It’s a long story - but we KNOW that he had planned the day and time ahead and the phone call wasn’t going to change his schedule.
"The struggle you're in today, is developing the strength you need for tomorrow."