The Samaritans and the Jews did not like each other very much. They lived apart and did not consider themselves to be neighbors. So by using this parable as His example, Jesus teaches us that everyone is our neighbor.
Nobody is excluded.
Yes, but God counts our love for others as if it were love for Himself. So when we love others, we are loving Him at the same time. And we cannot claim to love God until we first love our fellow man. The Pharisees obeyed all of the Ten Commandments, yet God was very displeased with them because they did not have love. The exact thing God wants from us is that we should love all other people as we love ourselves.