I suppose it's the poetry between the two for me. I love how they can connect with each other, almost like they can read each other's minds. I mean, Sanzo can see how Hakkai is doing by simply looking at his eyes, despite the normal smiling facade Hakkai puts up, and Hakkai and easily tell how Sanzo is by just how the man acts.
I think they get alng with each other awesomely, and that's how couples that have any sort of future should be in my opinion.