What are the reasons that turn a person from a cheerful, optimistic, and life-loving individual into a mass of silence and isolation?

Losing your best friend ever due to suicide when you could've helped...

It's not your fault. If you could help them, you would do it. So don't blame yourself.

I was completely ignorant and I didn't notice or react to the signs he showed while talking to me...

Another reason not to feel guilty, if he needed anyone to rescue him, he would say it directly. Look I'm a Muslim so in fact I haven't faced something like that before in my life. Suicide is forbidden. Whatever you go through life, everything will be okay one day. I just want to tell that feeling sorry for something you couldn't handle because of lack of your information (and it's completely not your fault) maybe will ruin your life or at least make ir worse, and I don't think your friend wants that, don't you?

I know but it's a big story honestly and it makes me feel bad till today, and the fact that he was my irl best friend and so was I to him, makes it even worse for me to face this fact.