There are only a few reasons an instructor might opt to not show you spins. It may be the type of aircraft you're flying. If you're training in a 172, your instructor probably knows that it doesn't spin all that well (you pretty much have to force it to spin). If you're training in a 150/152, and he still won't show you spins, I'd question his instructor ability (mainly his comfort in the aircraft). I think there are a few instructors out there who won't show students to spin simply because they don't want the students to get so comfortable that they go out and try it on solo's.
I've had students in the past who would, after countless demonstrations and explanations, refuse to fly with their feet actively on the rudders through stalls. To counter this I'd let them demo a power on stall and let them get into the inital stage of the spin where the wing drops, almost always left. After a while sitting there with my arms crossed, watching us hurdle toward the dirt, laughing hysterically as they flounder, I'd take the airplane and (as many of you know) do nothing and we'd come out of the spin. After a few times of this the students would always stay coordinated through the spin. I think there's value in teaching students to not fear spins but it needs to be taught at the right time. Teaching even a commercial student with 100-125 hours to "spin" is only a recipe for disaster. Everyone should know how to get out of spins but, should be taught that it's NOT a good idea to go out and do them just for the hell of it. There are very few if any students who really know all the aspects, (CG, power-on vs. power off, etc.) of a spin. Teaching students how to avoid spins is a much more effective teaching strategy than teaching it as a fun, wow your friends, type of thing.
After all this rambling, realize that your instructor probably doesn't want to teach you how to spin because maybe you're not ready to go out and perform them on your own safely.
In the Air Force we did spin training extensively in the Tweet. After 50 years and many testing the Air Force has a spin recovery maneuver that will recover from "any" spin. This recovery was so memorized and rote that a monkey could do it. Outside of a regimented training program, spinning is dangerous. We were prohibited from intentionally spinning while solo. But, if it did happen we could damn sure get out of it.