People commit suicide when their pain is bigger than their resources for dealing with that pain. A lot of times their reason is something we might think is a stupid reason, like they lost a job or a boyfriend, or someone teased them on the Internet. Sometimes their reason makes sense to us--they are in horrible pain from an illness and they're going to die anyway. Or they're about to be captured and tortured.
If you know that an illness or an enemy is going to kill you anyway, but first they are going to disfigure you and make you suffer horrible pain, it makes sense to try to escape the pain.
Is it a solution? No. Of course it's not. Nobody knows what's after death, if anything at all. Are people who do it normal? Not at the time, no. But they're not always crazy, just scared and in pain.