Never be ungrateful. Because; 
What would happiness be without sadness?
What would rich mean if poor didn’t exist?
What would be beauty without ugly?
I can’t exist if you aren’t there.
I can’t be sad with you as a friend.
I couldn’t know what happiness is if I haven’t been sad.
Could we tell what’s normal if nobody was mad? 
It couldn’t be easy without hard.
So never be ungrateful.
At least that would be a start.