When one decides on suicide they are deciding to play the part of God. And I personally don't want to act in defiance of God knowing that I am going to have to face Him very soon. Sure, life is tough. At times it seems that it would be a whole lot easier if we just ended things. But we are not thinking clearly.
Suicide is a faithless act.
And when a non-believer commits suicide they are in essence eliminating any chance they have of heaven. When a professing Christian commits suicide one has to wonder about the validity of their faith. But let me be real careful here. Some soldiers took their own life in the heat of battle to be kept from being captured by the enemy. King Saul fell on his own sword.