Very good thread!
Now, to the question.. this is interesting, because for me, it is pretty difficult to separate factors of martial arts training. It's almost like trying to separate and name every flavor and taste from a delicious birthday cake

) In other words, the entirety of martial arts makes me happy - kata, mokuso, kihon, sempais and senseis, dojo etc... I could go on with this list. Briefly, everything what you can find in MA training.
BUT, to answer your question properly I made a deep analysis and found three main factors that make my training enjoyable:
1) The development (mental and physical progress, growing up as a better person)
2) Learning techniques and their principles
3) The passion itself for martial arts