Why commit suicide ?

Is it that they are cowards, can’t face any problems that they encounter and seek an easy way out?
What bad might have happened to them that they don’t want to live anymore?
Do you think there can be a reason big enough to justify suicide?
I myself have had suicidal thoughts,what could it be that has been holding me from doing it?
What could possibly the best thing a suicidal-despo should do?