These dogs all seem to be smiling. Mouth relaxed, tongue hanging out, ears moving freely around the head, wagging tail ,and perhaps even wagging their whole body!
Dogs come to people when they want attention and love. A good idea when you're approaching a dog you don't know; Wait for the dog to say "Hi" to you. If they don't choose to, don't take it personally and insist. Remember that dogs, like people, are all different. Some may have not been socialized at an early age, have fear or aggression issues, or quite simply, prefer to be left alone.