Eating out & Friends

It's Thursday night. We had completed four long days of work. I was exhausted. Our ex-coworker friend called and invited us out to dinner along with our other ex-coworker M. M has an expense account and is always treating. I being the stubborn-so-wanna-be-righteous gal feel we should not be exploiting M's expense account especially when we are not giving her any business, even though our company does a lot of business with hers. So I alone decided not to go and just headed home to my cats and some peace and relaxation. My husband went and had a blast eating and drinking with them for 2.5 hours for free. Though I enjoyed my time alone thoroughly, I could not help but keep analyzing why I really chose not to go. Is it really because of the dinner on the expense account? Or is it because of the conversations on their life stories I did not want to sit through? Or is it because we see them too often? (No, could not be, I did not go to the last 2-3 invitations...) Or maybe I am just too tired and really did not want to socialize? Or is it because we never do anything else together besides eating out and that's getting old on me? Or is it I treasure my time after work at home ? I could not figure it out. But I think all these possible reasons add up to become a compelling one to say 'No'. Or I am just anti-social in general.