Their personality. If they are kind and cheerful they will show it without even realizing. They enjoy seeing others happy and bringing their mood up no matter who it is.
People with poor attitudes tend to share common qualities. Enjoy making people angry for no reason. Attacking people with unwarranted comments. Saying they want to beat you up randomly. Lying, blaming people.(You know who you are)
I genuinely believe your mood reflects who you are. If your around successful people all the time your chances of becoming successful are increased. And the same goes for happiness and personality.