I think it's human nature to often wonder, "what if?". It's probably one of the most common human conditions to wonder how a situation could have been altered by one or two small changes in past occurences. Simple things, like, "If I hadn't turned down that street to avoid traffic", or, "If he had brought my food on time so I wouldn't have been late." More often, it's complicated events like, "If I hadn't chose that major", or, "If I hadn't married this person." Usually, it's pretty harmless day dreaming that doesn't really mean anything. Regret, on the other hand, well that can be an awfully tough thing to deal with.
Regret, I think, often comes with unhappiness. Or at the very least, an unsatisfying situation. It means that something happened, either planned or unplanned, to lead to this moment that is so far from what was intended that a person wishes they could go back and change it. The problem is, even if you could change that one thing, it would lead to so many other things being different that the result would be unrecognizable. What makes it hard to deal with is that usually regret comes from a decision that the "regretter" caused themselves.
A self aware person will typically recognize that, I suppose, and say to themselves, "Well, I have only my self to blame." But a person who has regrets is probably the same kind of person who looks to things that happened to them and places blame there. As the saying goes, your attitude is controlled 10% by what happens to you, and 90% by how you react to it. Why would I regret something I cannot control? And so, I think regret also goes hand in hand with a persons need to forgive themselves for a reaction to something that steered them to the place they are.
I have a hard time saying that I regret something. But then again, the things that matter most to me I find to be blessings that all at once keep me happy, content, sane, hopeful, busy, sympathetic and empathetic, healthy, entertained, and probably most importantly, thankful. So, why would I regret? Because if I regret anything, then all of that could just as easily have never been part of who I am. Even if I continue to wonder, "what if?", it's just a harmless daydream that has nothing to do with right now, nothing to do with where I am, and nothing to do with the way my attitude is turned into actions that bring me to the next place and the next time something comes up that may cause me at best dissatisfaction or discomfort, or at worst misery, I can simply smile at the thought that it could all be very, very different, and go on trying to make it better.