is depression the main reason for suicide?

HEK

How can anybody tell if a fellow coworker / friend is depressed. So far in my company I have seen 4 suicides from people who laughed and looked so "normal" but apparently had serious issues in their personal lives that made them do what they did.
Personally if it's the time that you choose to end it all and end your suffering think about it, seek help and if all fails at least take the time to say good bye to those who love you.
It tore my heart to see a little boy brother of this young guy who worked with me to see him cry out loud at his wake, all I could do was just cry with him and when giving my condolences all I saw was a blank stare from the family.