Both Hugo and Ray LOVE being able to do these things by themselves without any help from humans. Such small things, and such a fleeting moment of happiness. Something that we wouldn't give a second thought to.
I think it's the complete autonomy of the act that makes them so happy. Of course, in Hugo's case it may be just the fact that he gets to help himself to more food. But in Ray's case? Is it being trusted enough to be turned off-leash for a moment? Is it having a home that he likes going to? Or is it just knowing where the front door is that makes him so happy?