Just like any other feeling, it's the one we come across often in our lives. Majority of its impact depends on how you take it. Some people aren't enough emotional intelligent. But we all understand it in our own way.
We regret about missed opportunities, unfulfilled dreams, unchased romantic affairs, etc. Nonetheless, on a personal level we often miss to show it, after all why should we be telling the world about this. We see it everywhere though. We see people and we see their regrets in their eyes. Now, it's okay to see someone disappointed by something and is regretting as those things and situations are not there anymore. But when people are disappointed by another person and they have to see that person frequently, it's very important how you show your regret to that person.