Some people, will regret for others. It's just a reflex we have in society for some reason that if something bad happens for someone, you don't empathize with them, you regret for them. It's part of the reason why if you had a bad thing happen to you, you feel worse because as much as people saying sorry to you (even though they have nothing to apologize for) may help, you see them again later being fine and blissful. Someone saying sorry to you won't do anything, as the "sorry" just shows a brief feeling of grief but because nothing happened to the person delivering the sorry, so it is a hollow one.
When you regret over something for yourself, what do you feel? For most it's anger or a profound sadness. Not a sadness of missing a possession, but of insufficiency. Not able to accomplish something with in a deadline, but the deadline wasn't told to you. So you feel cheated. Most of the time the task was so simple, that you felt you should've been able to do it extremely easily.
But why were you not able to complete it? Are you incompetent for not being able to complete it? No of course not! Still then, why could you not do it? You can't get that answer no matter how hard you try. Humans not getting answers is pretty bad, we don't like being withheld information, so we either make it up ourselves or beat ourselves up over it in hopes that it we would find the answer somehow.
But obviously that doesn't work with, and it sticks with you until you die. It all comes down to moving on, and learning from the regret so you don't make another one.
Thanks for reading. I don't really know where I was trying to go, I just wanted to write something relevant to what was going on with me today.