I think I have only one regret in life, and it is this:
When I was in college, there were these kittens (and eventually cats) behind the pizza shop where I worked. I would buy a big bag of cat food and keep it in my car and feed them every night after work. If I didn't work that night, I would come back. They would be waiting for me every night at around 11:30.
There was two of them in particular, named Boss and Dewey. There were other cats, and we did rescue a few of them, as well as other cats who would just come and go. But Boss and Dewey were always there, and I had been feeding them since they were little tiny kittens.
I always talked about bringing them home with me, but never did. I was worried about having 3 cats, since I already had one already. One night though, I tried to catch them anyway. They would let me pet them and pick them up, but when I put Dewey in the cat carrier, he panicked. I felt really bad, so I let him out.
A few months or so maybe after that, Dewey was hit by a car. I don't think I ever felt more rage in my entire life... especially because what apparently happened was some asshole in a sports car was flying through the back of the little shopping strip, where he shouldn't have been anyway, since it was restricted to only trucks and employees. What really broke my heart though was one of my co-workers at the time said Boss just sat next to Dewey's body for a while.
I felt so awful knowing that if I hadn't hesitated, I could have saved Dewey. I made up for it by bringing Boss home that very same night, though. I don't regret that, at least.
That was about 8 years ago, and he's been a really wonderful, super affectionate cat.
But I'll always regret not saving his brother when I had the chance.
This is Boss, by the way:
Beyond that, I don't really regret anything. There are definitely things I wish I'd have done, but at the same time, I know that if I had made those choices, things wouldn't be as they are today. So... I don't regret it at all, seeing as I am otherwise pretty content with my life.