God chooses when we are born and when we die, not us.
Pain, loneliness or depression might be for a reason: Christians believe that by suffering they become closer to God, and share in the suffering of Christ. They believe God sometimes allows suffering so we can learn. Suicide is a refusal to learn the things that God is trying to teach.
Suicide is selfish: it causes much pain to those left behind. They may blame themselves. It is harder to get over the death of a suicide, than someone killed in an accident.
“You shall not kill”. Suicide is murder of yourself.
“Don’t you know that you are the temple of God?” 1 Corinthians 3:16. Christians have a responsibility to look after and respect their bodies, because God lives in them.