Obsessions

I took my son to a pre-prom party tonight. He went to the alternative prom, and is also going to the Junior prom. The party was at the Bishop's house. He has a nice house and yard with a pool. They have a pug dog named Bruiser. It's so funny to see this little dog running around and hear people call out "Bruiser!" That name totally fits him! I walked out to the back yard to take some pictures of the kids. As I was going out the sliding door, I noticed Bruiser was standing staring at the screen door. At first I thought maybe there was a bug crawling there or something. I asked the Bishop about it, and he just replied, "I have a psycho dog." Later, as I was leaving, I came back and saw Bruiser once again standing and staring at the track where the screen door slides. I asked the Bishop's wife about it. She said, "He is obsessed with the movement of the screen door. He waits until someone opens or closes it, then he runs." I decided to humor him, so I closed the screen door. Sure enough, Bruiser twirled around a couple of times, then took off running around the yard. My question is, how does a dog get obsessed with the movement of a door? lol