Scenario I: I work very hard. But my boss has his own favourites. Is there something I haven't done?
Solution: When a boss plays favourites, it definitely hurts.
However, do first analyse whether there truly is something you haven't done.
Make a checklist of duties, functions and daily chores you are responsible to perform, as well as the benchmark expected of you.
If you feel you are doing everything that is expected of you, seek your boss's guidance along the way and communicate more to improve your relationship with your boss.