I've definitely had times of depression in my life, but I've never considered suicide. One really depressing thing to me is mortality. The fact that we only get a certain number of years, then that's it, and there's nothing you can do about it. Whatever you don't manage this time around, you never will. 80 or 100 years (or whatever) doesn't seem like nearly enough. So it'd be pretty ironic for me to deliberately cut my life even shorter.
Re. what others have said about how to handle someone who's considering suicide, I agree that you shouldn't make them feel guilty, or like they're a bad person for considering it. I do, however, think that it's good for them to be aware of the devastating effect it will have on other people if they go through with it. They may have lost a sense of responsibility to themselves, that they don't owe it to themselves to go on living, because they don't feel they're worth it. Sometimes being needed by someone else is what gets them through it.
On another forum once, I saw someone post that they had made the decision to commit suicide, and were wondering whether it's better to leave a note or not. They asked that people please not sidetrack the discussion with trying to talk them out of it, because they had already made that decision, and just wanted opinions and advice about the note. One person replying suggested they should write the note, and leave it where the recipient would find it in plenty of time before they committed the act, to see what their reaction would be. I thought that was an interesting way to reply. (Several people had already replied trying to talk them out of it, btw.)