Please just explain

Ever go through life thinking things are one way- living in a bubble only to have it popped? Please explain that to me! Why do we live so blindly and fail to see the signs around us that point to things that are right in front of us? Often we are so quick to point out anything for our friends and family, yet when it comes to our own "backyard" the elephant seems to elude us. I totally don't understand. I guess we fall victim to that adage that nothing bad will happen to good people, or maybe we simply select not to see the bad around us in hopes for change. I for one hate to think that people hurt me on purpose. That is just plain mean. Why would anyone knowingly do something that in turn would cause damage to my heart (yes I am aware it's more like my brain- read that blog for heart vs brain reference)?  I would like someone to explain just explain the "why" behind doing something knowing the end result.... cause to be honest- I don't get it!