
Just over two months ago, I started going to one-to-one agility lessons with my younger dog (6yrs old). From the first time we went, on arriving home he would be exhausted for the rest of the day, but awake and staring at me constantly, just like you describe. I wasn't sure why either - it was like he had a new respect for me, or perhaps hoping we were going to do some more!
11 lessons later he doesn't do it, and is much less exhausted (we are both a lot fitter, lol) but if I put up some weave poles in the park he is instantly in working mode and would practice all day if I let him.