After 21 years and 330k miles of riding, I too am never bored on my present 500cc 40hp bikes. It's still more fun to ride a slow bike fast........
I started out like most do, bought a CB400 first bike and 6 months and 6k miles later I "needed" a CB750 to keep up. Now I ride my GS500s much faster than I ever rode that CB750 for the 80k miles I had it 20 years ago.
Now when I hear someone say " I need a bigger bike," in most cases I just smile and say to myself "you need to learn to ride the one you have first," like I needed to back then.
With gas prices over $2 a gallon and probably going much higher in the future, I really appreciate the 60 mpg I get from the GSs. Running it hard and fast can drop it to 50-55 mpg at worst. I hit reserve at about 220-240 miles allowing a full day and somestimes two between gas stops, except when traveling.
I am frequently asked why I don't get a bigger bike by those I don't know well. Most times I just kiddingly tell them "I wouldn't have anyone to ride with if I had a bigger motor."

407,211 miles in 30.1 years for 13,528 miles/year average. Started 7/21/84, updated 8/26/14