Regrets after Breaking up

Has anyone ever regretted leaving someone just because they had found someone who they thought was better but in the end it didn't turn out to be that way, and there is no going back? Why do some people knowing that they want to leave that person start acting cold instead of admitting that they no longer want that person in their life? Wouldn't it be much easier to just be honest than to hurt the other person more by acting cold and staying with them a little bit longer.