WOW! I thought I was the ONLY ONE who got pissed off about this!
My boyfriend use to do it to my on our group rides... he would sprint off... then slow down to let me catch up... over and over. Constantly looking behind to find me... and slowing down.
One time he even stopped and waited for me on the side of the road.
I knew the route though and was FINE to do it alone. He made me feel very inept (? spelling), and I would get really pissed at him.
Well, I don't do that ride anymore. I let him go it alone... and he has since sped up and kicks major butt... without me being there to slow him down.
Anyways, I totally understand why you were pissed. Sure, it's OK to slow down if you are trying to keep a group together and the back people don't know the road/route... but you two can do your own rides... just fine.
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather, to skid in broadside thoroughly used-up, totally worn out, and loudly proclaiming: WOW WHAT A RIDE!!!!"