Why is it always TRAGEDY that brings the world closer together? Why is it NEVER something joyful positive beautiful?

Because tragedy makes people realize that they aren't really in control.   When tragedies aren't occuring, people/nations fall into a false sense of self-pride; that only they matter (generally speaking).  But when tragedies occur, people see that it's really not only about themselves.  That we're all this together.  
Whether it's a tornado, flooding, a snowstorm, a hurricane, an earthquake, or a global pandemic, it seems we need tragedies to keep us from getting overly complacent.