I think Paps knows Sans so well he knows exactly when his brother needs someone despite wanting to be left alone. When Sans needs a hug Paps gives him a hug. EDIT: Fixed wrong pictures order Dis is ART!
This is exactly what happened to me. I was scared to battle it because I didn't have any poke balls but then it said that it just joined my team and I screamed. And while This happened my dads friends were over so it just became weird