We all want to be happy.
Maybe the explanation can be explained through the quantum world. Maybe my atoms are attracted to atoms that vibrate at higher frequencies.
From an evolutionary perspective I want to be around happy people because happiness is a sign that all one's basic needs are met and thus I subconsciously want to be around those who have a better chance of survival.
From a psychological point of view I understand that being around positive people will influence my own thoughts and perception of the world thus ultimately affecting how I live my life.
All of these are just different windows through which we see the world.
The conclusion will always be this:
Happiness is attractive.