1) If I'm flying with someone it means I enjoy their company. Odds are I will enjoy the company of my companion in close quarters more than the company of a random stranger.

2) If I'm flying with my GF we can put up the armrest if in coach and sleep much more comfortably next to each other rather than trying to avoid physical contact with a stranger. If in First I'd much rather have her climb over me to the restroom instead of some random dude. Or vice versa. Even if flying with a friend it's much more comfortable enjoying the flight not worrying so much about invading personal space and trying to share a tiny area.