I still find it interesting when I first describe my "railfanning" hobby to others.
The idea that I'll go and spend time looking for trains or taking photographs of them seems "strange" or odd.
Well.. it's just something I enjoy doing. Why?
The easiest answer is I like trains. I have thought they were cool since I was a young kid. My dad built us an HO-scale model railroad in the garage (we didn't have basements in California, and we used to run them around the plywood with a few buildings and a "Plaster of Paris" mountain in the corner. I really enjoyed going to Balboa Park to visit the huge model railroad museum there (which still exists http://www.sdmrm.org/ ) and also our trips to Los Angeles on Amtrak. These days, I enjoy photographing railroads too. They are never the same and make for interesting subjects.
Also, I like history. When I look at the names on the side of boxcars you can usually see a few from railroads that don't exist any more. Some times they might have merged into a current railroad, but many times they have just been abandoned. I see those names and it reminds me of their stories.
Finally, I like transportation. Trains move things. Even when they are empty they are on their way to bring another load. Be it a hopper full of coal for a power plant, a tankcar full of chemicals, Amtrak train full of passengers, a container load of toys for Christmas, or whatever. Without trains, America would stop.
So while other folks get excited over sports, hunting, running, or whatever; I'll stick with my trains. See you by the tracks.