I don't want to sound hoity toity, but I want you to know my background before I give you my theory on this subject. My Master's degree is in Zoology, with an emphasis on animal behavior, and here are my two cents on the subject.
Mammals, including humans, have a chemical in our brains called Oxytosin. It is released during cuddling, sex, nursing, courting, falling in love, and other social interactions. This chemical makes us bond with others.
Birds have the ancestor to that chemical, mesotosin, and it is released in their brains during social interactions. This is what makes the birds want to be with their flock. The chemical must make them feel good, or they would avoid the behaviors that cause it. Since it is similar to our chemical that makes us so happy, I think we can assume that they are feeling the chicken equivilent of "happy". When they follow us, and want to be groomed by us, it is because they accept us as one of their flock. So, we probably cause the release of the happy chemicals.
I know my birds love me in their own little chicken version of love.