Perhaps in addition to Spinning you undertake other forms of exercise- may be in a class setting or alone? If you do the doe you find yourself comparing them with Spinning. Take Metafit for example very intense but mercifully short whereas with Spinning you get a longer time to enjoy the ride.
Or may be you are it for the satisfaction of completing the class or as proof that you can ride a bike when you do not wish to do so out on the road?
How do you feel afterwards when showered and clean? Pleased? Relieved ? Relaxed? Does Spinning make you feel happier? Like your body has released some endorphins or something similar to make life feel a little brighter.
Or perhaps it is the social aspect of meeting people at a class that attracts you and that if this part you absent you are not sure you would bother. May be you like something that involves a more meditative style rather than the frantic activity of a climb or set of jumps. What ever you like best then it is great if Spinning is part of your weekly program of activity. If it is not the why not give it a try?