On my own I can go a long time and stop where I mean to. When my partner is the one doing it, I am gone in a couple of minutes and I have twice ended a session about half an hour earlier than either of us intended.
She finds it funny, which is fine. I find it slightly humbling given how much practice I have put in.
What is actually different? Is it just that I am not in control of the pace, or is there something more to it?