I have 6 bikes, one modern one & the other's span from the mid-1970's to the early 1990's. All different geometries, different wheel sizes (27", 700c, 26"). Also all different types of tires. So yesterday I was riding my 1992 Hardrock Ultra around (I replaced the bottom bracket with a new Shimano UN55 and installed my, new to me, Shimano BC124 crankset, and adjusted the front dereilleur). I intended to just ride it around to test the shifting. When I was "done" I'd pull up to the house, slow down, then go for another loop. After I was sure I was done, pull up to the house, slow down then think "Just a little more". I kept riding and riding - not far but around the park and the neighborhood. I finally realized that riding this particular bike is FUN! It wasn't fun for any particular reason - it didn't bring back any particular memories of having fun on this specific bike (I've never owned one of these before), it wasn't the weather, it wasn't my pre-existing mood. It was just simply fun.
I know many of you have multiple bikes, and that many serve particular duties (touring, commuting, etc). So....What makes riding your bike(s) Fun? Is there a particular one that is more fun? What makes it fun?