I have been pondering this for some time and have brought it up in comments with
chomijiand
telophase, but I have decided that we have been misinformed. Goku is NOT a monkey. He is, in fact, Sanzo's pet dog. Specifically, the stray dog who followed the grumpy guy home one day and became obsessively devoted after being fed.
Observe:
1. "Sanzo Sanzo Sanzo!" "Buy me that bun!" "Feed me! Feed me!" *tug tug tug* is an AWFUL like 'Yip Yip Yip!" *slaver slaver* *paw paw*
2. Like the most faithful of pets, Goku is almost literally blinded to anything not in Sanzo's immediate radius, and only notices other things that aren't directly related to Sanzo if Sanzo isn't there.
3. The only things to distract Goku from "Sanzo Sanzo Sanzo!" are basic physical needs: Food, sleep, fight.
4. Gojyo: Gojyo pisses Sanzo off just by existing and upsets Sanzo's equilibrium. As pets are highly sensitive to their masters' moods, this causes Goku to rabidly protect his master by always fighting with Gojyo.
5. Hakkai: Hakkai is good for Sanzo's peace of mind(being the only non-annoying person he knows) making Sanzo calmer. Therefore, Goku adores Hakkai.
6. Kougaiji: Kougaiji has virtually no affect whatsoever on Sanzo's mood beyond the general irritation that everything in existence causes him and thus would not even register to Goku, save for one thing: Kougaiji PLAYS with him, and more than anyone else will. Therefore, Kougaiji is Goku's bestest-estest friend ever.
7. Lirin: Lirin essentially is a female Goku(hence Kougaiji and Sanzo's tolerance of the other's pet) and she invades his territory and dares to touch his Sanzo and get food from him. Thus, Lirin is his mortal enemy and he must defend his territory.
8. I can't recall if there's any evidence of this, but come on, does anyone have any doubts that if you threw something and said "fetch" he would(unless you're Lirin or Gojyo) or that if you tossed food at him, he'd catch it with his mouth?
Yes, it is entirely possible that I posted mostly because I finally got around to uploading a
Saiyuki icon and wanted to use it.
PS-
chomiji has a point...Goku and Lirin should grow up and get married. Not because I ship them or anything, but can you imagine the reactions of and conversation between Sanzo and Kougaiji? Not to mention everyone else...