Recently kicked off our honeymoon with UAL, flying LHR > SFO. I dressed very smartly, asked very politely, and got very politely declined at both check-in and the UAL information desk.
Undeterred, the missus asked at the gate and promptly came back with a pair of C class boarding passes. Then again, she’s much better looking than me.
I suspect its partly attitude & appearance, but mostly luck of the draw.