On seeing a happy, loving couple my first reaction is always one of teary admiration. My second, which arrives very quickly, is sniffy suspicion.
"Can they really be this happy?" I wonder. "How is it possible after 18 months? Aren't they bored of each other yet? She's a lot better-looking than he is, so, well, at least one of her eyes must wander." These are just some of the life-addled thoughts that pass into my consideration.