Some time ago, in 1984 E.O. Wilson, a biologist from Harvard proposed a fascinating hypothesis which he called "Biophilia". And told us that it was an innate interest of human beings by nature, and why do we have that interest? Well, because it is adaptive.
Imagine our ancestors in a natural environment, full of predators and elements of nature that could end his life. Can you see yourself there? Pay attention now at the sounds of birds singing, the wild horses grazing in the distance and the sun shining in the sky. You know that all is well and nothing threatens you. Suddenly the birds cease his singing and horses flee frighten. All your senses are alert and you get ready because you know that something bad is coming...
A similar evaluation of the nature, watching animals in a resting position or in state free from excitement that suggests the hypothesis biophilia as an indicator of well-being and safety for humans to observe.
Can you now understand, why do you feel good with your cat on your knees or because you're with a sense of security and peace when you see your dog sleep on your bed?