There are many kinds of pain that may lead to suicide. Whether or not the pain is bearable may differ from person to person. What might be bearable to someone else may not be bearable to you. The point at which the pain becomes unbearable depends on what kinds of coping resources you have.
Individuals vary greatly in their capacity to withstand pain. When the pain becomes too much to bear then certain individuals consider killing themselves.
When individuals find themselves in such a situation it seems rational - to them - that the only way to end the pain is to kill themselves. The only was to prevent suicide is to demonstrate that there are ways to challenge the individual's negative thought process.
If you, or someone you know, is thinking about suicide contact the
Samaritans. They have a telephone helpline and also offer advice by email.