Why do I like the Denon 103r so much?

It's also possible that the cart just clicks with your arm better than the Shelter. Arm-cart compatibility is a big thing, just as speaker-room compatibility. When you find a pair of speakers that just work in your room, it's like a revelation. Sounds like you might have something like that going on. Enjoy and don't screw with it too much!

I have only used a Denon 103pro. Undoubtedly it had the most unadulterated tone and timbre I have heard.

I was using a VDH retipped Crown Jewel (aka Shelter 501) when I got my first 103R - and much preferred the Denon for its involvement and superior tone and texture and presence.
With this cartridge I find myself appreciating the music itself and how its being played - rather than the hifi qualities.
That said, the SoundSmith retip can greatly improve the top end (especially) and clarity in general. Focus, separation, recovery of ambiance and other soundstage dimension/details are also much improved. The stock 103R is a very good tracker, but the line contact retip tracking is significantly better. The musical qualities of the stock cartridge are essentially retained, though of course it does sound different.

Tobes, I very much like what I am hearing you say re the Soundsmith retip. After some consideration, and living in England, I made the practical decision to send my retip/cantilever upgrade of my Zu Denon 103 (and Transfiguration Orpheus) to the Expert Stylus Cartridge company just 20 miles from me, rather than across the Atlantic to Soundsmith. They have fantastic positive feedback, and I am very much looking forward to the big shoot out btwn the Zu 103 and Orpheus (which is 4x the price). I suspect the Zu may lose on points, but there'll be no definitive KO. Maybe there'll be a shock win for the Pretender!

Spirit, at some stage I will send a 103R to Expert Stylus so I can compare with the SoundSmith version. The different stylus shape will certainly have an impact on the sound. I've heard some say the 'Paratrace' stylus (similar to VDH's profile?) used by ESC is superior and sounds 'better'....but I've never read a controlled comparison. Since line contacts are fussy beasts to optimise, it would be easy for one to be slightly less well aligned for geometry/azimuth etc and the 'win' would go to the other.
Plenty of users with the ESC version and, like you say, it has great feedback. I'm sure you'll enjoy it.