why does holden feel "lucky" when he remembers throwing around with robert tichener and paul campell?

Holden seems to cherish those kind of memories:

I was lucky. All of a sudden I thought of something that helped make me know I was getting the hell out. I suddenly remembered this time, in around October, that I and Robert Tichener and Paul Campbell were chucking a football around, in front of the academic building. there were nice guys, especially Tichener. It was just before dinner and it was getting pretty dark out, but we kept chucking the ball around anyway. It kept getting darker and darker, and we could hardly see the ball anyway. It kept getting darker and darker, and we could hardly see the ball any more, but we didn’t want to stop doing what we were doing. Finally we had to. This teacher that taught biology, Mr Zambesi, stuck his head out of this window in the academic building and told us to go back to the dorm and get ready for dinner. If I get a chance to remember that kind of stuff, I can get a good-by when I need one - at least, most of the time I can. As soon as I got it, I turned around and started running down the other side of the hill, toward old Spencer’s house. He didn’t live on the campus. He lived on Anthony Wayne Avenue.

Holden has made a mess of his time at Pencey Prep, but even though he has failed classes, angered his 'fencing' buddies, and been asked to leave the school he does have fond memories. Before say 'goodbye' this incident comes to mind, it's a memory of good time there............. just throwing the ball around with friends into the evening; it's a good memory.