why do the one's that need the most love, reject and hurt others the most?

Have you ever sat and watched people, just went to a mall and sat there, people watched as it were?
Have you ever watched how people interact with each other, truly interacted, it's often painful to watch.
Why do we hurt the one's we "love" the most and treat strangers with the utmost respect?

Is it because we "know" we can get away with it, or is it truly a case of no one knows what goes on behind closed doors because they too are having issues and turn a blind eye...