I think OP makes an excellent point, as in it's probably inevitable, that at some point in any long term relationship, it will be necessary to "forgive" our friend, spouse, family member, for something we never expected to forgive, in order to continue the relationship..

What makes it so hard is it probably represents some kind of betrayal...something more deeply felt than the every day and normal "disappointments" we deal with and are better prepared for...

And it's a more likely a situation that we probably thought to ourselves; and other people may glibly think about themselves, "I'd never put up with that if that ever happened."

But some do. For whatever their reasons and hopefully for the betterment and benefit of each other..and a healthier relationship..