I think it's their sincere love for their owners no matter what.
They hold no grudges.
They don't care that you weren't in the mood to play 10 minutes ago and yell at them.
They just want to be near you, make you happy and just love you.
You are the most important thing in their world; you are their world.
You don't have to earn their love, they just give it - willing.
Just pure, no strings attached, love.
I came across these three images on pinterest recently, and each one melted my heart in a different way.