A Common Theme in All of Shakespeare's Tragedies

I have noticed that there is a common theme in all of Shakespeare's tragedies.  Cici was the thematizer for our group today and mentioned injustice as a prevalent theme in our scene.  I understood and accepted this view completely, and began searching for other themes myself.  I realized that Shakespeare's tragedies follow the same plot.  First, there's the initial situation that introduces the characters and the setting.  Then, the conflicts between two parties are discussed and tension between them builds up.  Finally, there is an initial fight, war, battle, loss or such and everyone realizes that the conflicts between the two parties has caused only tragedy.  Thus, I concluded that Shakespeare deliberately set up his stories to show that his two themes, trust and friendship, are the ways to live a happy and fulfilling life.  If you look at his tragedies, the two parties I mentioned earlier do not trust each other and are hardly friendly such as in King Lear when two characters (I forget who) disguise themselves.