Because you're less attached to strangers, less trusting of them than your loved ones. You see your loved ones more, will most likely see them in the future, and know the issue happened. It's closer to your heart. If you care enough, it's worth it to forgive - depending on the reason.
It depends on your morals. For me, it's if they're too selfish, condescending, and/or a liar. I can only take so much of that; but no lying. I can take honest mistakes. I don't mind listening to people vent. I don't care how smart you are. To know when something is unforgivable, you must know your own moral guidelines. "Do not impose on others what you do not wish for yourself." - Confucius Just make sure you're healthy and happy.
definitely happy...i know what i deserve. i divorced my ex because he cheated on me with craigslist prostitutes.
Because we have the tendency to manufacture expectations and assumptions about the people we love (how they're going to treat us, what they will say, what they are capable of and what they aren't), then, when those false constructs collapse or are bent excessively, we get hurt because it is our own pride in thinking we can 'know' someone that gets torn down. As human beings, we like to think we 'know' each other, but you could live every minute of every day with the same person for a thousand years and you still wouldn't understand them completely. Oh, and yes, I am stating that all heartache and heartbreak is a consequence of the victim's own pride being compromised rather than the trespass itself.