Is it stress or do you just do this?

During lunch, my friend told me she cut herself last night, and another friend of mine said she overdosed herself on meds. Does everyone want to end their life when they have so much ahead of them? I understand that they are stressed but suicidal thoughts aren't the answer. I'm just glad it wasn't too serious. My friend cut her wrist, while the other only got sick and is moody. I've never dealt with this till I started middle school. When I became friends with my best friend, she suffered horrible depression. She was always moody. I remember when she would have outburst of crying episodes during class and passing period. I've tried everything to cheer her up, everything. Now, her depression isn't too severe, but there are those episodes, because of "high school stress." Even I have that, but I don't just go and cut myself. I still remember when my best friend turned to me in the hallway and said that she was going to end her life. I was in total shock, and she walked away, leaving me to stand there. When I got home, I bawled my eyes out. She was my one and only friend that was ever good to me, kept secrets, never fought with, and we've been friends longer than any other friend that I had. She's my best friend and I don't want to lose her. I've asked outside help, and they always told me to tell a counselor, or at least a therapist. I kept the idea in my head, and kept it in there, in case she gets those suicidal thoughts. Now, my other friends are planning to hospitalize themselves! It's ridiculous, think before you act. I really want to help, but I feel like there's nothing I can do. They tell me not to tell, and I get that feeling to at least tell someone, and to get help. Why can't I help them? Why can't I save them from the Grim Reaper taking them away? Why can't I help them from going into a comatose? What can I do?