For me, any time I hear a cat purring, it makes me laugh inside and I smile and feel all fuzzy & warm. But if a cat is sitting on my lap, or is near my face, just hearing a LOUD purr just makes me laugh. I think the louder the purr, the happier the cat is. But there's also other signs of a cat being happy such as tail twitches, wrapping their tails around our legs, meowing (or as I like to call it: "chirping") and for me, what really makes me laugh and pet a cat is if a cat comes up to me (doesn't matter if it's one of my own, a friend's cat (or cats), or even a cat I've never met) and just flops over, lays there in the middle of the floor, just looking at me as if to say "what are you doing standing there, looking at me HUMAN?! PET ME!!!!!!!!!!!!" and purring like crazy. They know how to charm us into doing things for them, don't they?