In a small way, when people tell me they liked the food I made them. I completely agree with you on the fun pseudo-alchemy of cooking, especially off-recipe. To me, food is culture, art, and both of those things are improved by sharing.
In a bigger way, when I make some serious progress musically. Being able to set down my guitar or uke after really getting into a riff, or doing some complex vocal piece with confidence, gives me a huge rush.