Relationships are such a complex and fragile thing. Even the relationships that have endured a lot are just as breakable because every relationship has its weak points. If you know where to strike the blow that hits the hardest, it cracks. And all it takes is one blow for everything you've invested your time, effort, heart, and soul into to shatter in an instant. Even if you manage to repair that, the cracks will still be there. The pain and the experience can't be washed away. The scars become a part of you.
I believe that when a person realizes the true vulnerability that exists in relationships that that's when it strengthens because the weaknesses become appreciated and acknowledged. So many of us don't realize how easily we can hurt someone. We're shocked by the sensitivity of others, yet become easily heartbroken just the same when it happens to us. The best protective measure any of us can take is to realize that there is true danger even in the seemingly smallest of acts and that those "small things" can be irreversibly damaging.