Why does it always take something bad to happen for me to realize how much I really appreciate something. Or why does it always feel like there should be more I could have done to avoid that bad thing from happening?

Do we always feel like there's something more we should do? If we feel like we've already tried everything then why does it feel like I haven't done anything at all? Does it mean I have tried enough? Or what I call "tried everything" is just useless actions that no one is going to take notice of anyway?

Why is it that I have this sense of guilt about this? I feel like I should have paid more attention? Should have just done something more, but what is that something?
What could have been done to prevent it? What could have been done? Someone just tell me?