S/he can win if you and s/he legitimately see things differently and disagree, but usually – you don’t.
You and s/he even agree politically most of the time.
You also care about the same people.
S/he’s the last person in the world you’d need to argue with.
But sometimes, like a kitten wrestling with string, s/he might start an argument just for fun. Sparring for sport. S/he tells herself she’s setting you straight about something, but s/he’s just arguing recreationally; people get high from argument.
Part of the reason you love her is that s/he’s so witty, s/he’s good at argument and almost always wins. But s/he can’t win against you! You love her too much. It’s the ground you stand on.
You don’t need to get high on an argument; you’re already euphoric because she’s got you so full of love. You can hardly remember what the argument is about, because you’re distracted by how impressed you are. No room for ulterior motives, you’re filled up with love. That sincerity causes all your arguments to be valid before you even make them. You don’t even have to say anything.