Nothing to live for....?

I know it's been a while since I last wrote on here, but I've been pretty busy with work and personal issues. One of which is what I'm going to talk about now...sort of. This week has been pretty depressing and confusing for me.
I have a friend whom witnessed a friend of hers commit suicide. And another friend of mine caught her husband attempting to commit suicide. What drives someone to that conclusion? I know this is an extremely touchy subject and I'm in no way trying to offend anyone or make light of the situation. It is a very difficult and serious subject matter. I just, personally, can't seem to understand why or how someone comes to that particular decision. I understand life is difficult and everyone goes through things differently, but when does it come down to a point where you feel as if you have nothing to live for anymore? What goes off in your mind or wrong in your life that life seems unlivable?
My friend Lisa's* friend Tina* was having a difficult time. She had just lost her youngest child to a drive by shooting and she was just at her wits end over it. She has 3 other children, but the lost of her youngest, her only daughter, tore her apart. The oddest thing for my friend Lisa was that she felt angry that Tina would do such a thing. She had 3 other children; did they not matter? The other kids were just as angry; they felt like Mom didn't love them. I kind of understand Tina's possible thought process of her actions; things were, to say the VERY least, wretched. But what I don't get is was the wretched portion so horrible that it drastically overshadowed the good in her life; her other kids, her fiance, life in general?
My friend Lauren's* husband, Mike* was the happiest guy you'd ever meet. He was funny, smart, a great friend, even better husband and father. No one knew he was having a hard time, at least what we can only assume was a hard time. He had a great job at a promising company; had even just got promoted. He had his family. He was an avid volunteer for kids in foster care and in hospitals. Mike was a great guy who had so much to live for! What was the straw that broke his back and made him give up?
Maybe these questions will never be something that I will understand, but they're definitely something that I think about, especially now with all my friends going through it. I'm sure I'm not the only one either. I could be fishing for something that I don't necessarily want the answer to because it relates to people I care about, but that's also the reason why I'm questioning. Is there really an actual answer? I'm sure the are different reasons for every person, but wouldn't the underlying emotions and feelings possibly be the same?