I say there are a few reasons second chances, or even more than that are given. We all make mistakes and it happens on both sides, if not due to our character flaws, or through mis-understandings. Even if, or though we regret our actions, we cannot undo them, but we can make amends and even help others to trust again, if we are loving deeply enough and prepared to give a few chances. Are any of us capable of not making mistakes or in never hurting others? Are your loved ones not worth being given chances?
Experiences in life do change us, usually for the better if we are able to learn from our mistakes. If we don't, we are doomed to repeat them and to live with more upset and cause more than needs be.
Sometimes, even though we are hurt by the actions or non-actions of our partner, to be without them or to end up with someone that we don't have the same chemistry with, is futile and shallow and can be very painful. There are many people, male and female, who prefer the ups and downs and the drama than having a non-emotional relationship. For in the calm times, there is still real feelings of emotional attachment and belonging. We must not under-estimate that incredible human need.
Communication after an upset, where we try to understand each other and hope to improve the relationship, is extremely beneficial. No-one ought to just carry on the next day as if nothing went wrong, because something did, and there needs not to be the same scenario played out. At least, not the exact same one as it can get very boring.
There are certain times, AFTER a few chances or even before giving another chance when the worst happens making it a very bad move to give another chance, but that is another article.
I could continue for ages on this topic, but this will do for now.