Why do we offer our best behavior to "strangers" and not to those we profess to love?

Why do we tend to inflict our worst/crudest behavior on those we claim to care about the most? We behave better in front of people who, relatively speaking, don't mean anything to us and then regress around our loved ones. The way we dress, act, speak is often less respectful around family. We say we should be able to be "ourselves" around family, but why does that seem to mean our least considerate, sloppiest selves? I would think we would offer our loved ones our best. To some extent, we all seem to do it. What are your thoughts?