The life of a person who has lost everything is very different from a person who didn't have much in the first place. Whether it should be different or not can be discussed, but the reality for most people is that the 2 cases are very different.
Regret and despair are very real and play a vital role in life after a big loss.
I believe you and your parent commenter don't agree because you seem to be trivializing the issue a bit - in a way similar to how people who haven't been through big loss, or feel that because they got through it fine, so everyone else should also be able to do it as easily.
It ignores the inherently different way in which people deal with such circumstances.
> You have the power to not let your feelings be determined by external events.
There are smart people who are unable to exert enough control over their feelings (even when they know the above idea). People are diverse and often biologically (or culturally or both) wired differently, and people who are a little lucky in the ability to handle such situations better often find it easy to wonder "how idiotic can the person be to commit suicide on losing everything" - just because they handle it better.
ps: About people being wired differently, it is easy to see in certain aspects. Some people tend to panic more than others, and then there are those who have panic attacks under pressure. Then there are those who have an inherent bias towards one of either fight or flight.