[! !!minor lmk S4 Spoilers below but you've probably already seen/heard of the scene anyways!! !]
Quick reminder! That Tang has done the little giggle thing that everyone was losing their minds over a week ago twice actually!!
The time everyone already knows about was in S4 when he giggles as he nuzzles pigsy after getting him to return to his normal form
But the first time that I can recall is actually from S3 when he's trying to get monkie king to let him tell the backstory behind the samadhi fire, it's mixed in with small excited Tang noises that make it sound like he's about to explode but it's very much there
In conclusion!!! THE two things we know of that seem to make Tang happy to the point of doing the little giggle thing are 1. Talking about his heavily-implied-to-be-hyperfixation on monkie king lore and 2. Being reunited with pigsy. I leave you with that thought :3c