Summing up the fondness, the affection that he feels when he looks upon Ja'far is something best left for things that aren't words.
How does one explain 'friend' and 'partner in crime and business and all affairs' and 'love of his life' all in one term, after all? An impossible thing, all of it, especially when he is so used to Ja'far by now—used to his nagging, the smack of a scroll against the side of his head, the sureness and swiftness with which he acts in all things politic and battle, the way his brow softens around a child or the way his gaze lids in the privacy of Sinbad's rooms with the brush of knuckles against a freckled cheek—
(The last part, perhaps, Sinbad still isn't quite used to).
He's used to Ja'far, the flux and flow of things that happen when he is around, and it's nice, like that; nice in a way that thoughts don't require, especially with the guarantee that it will continue (because Ja'far's words are always a guarantee, always, even if that only strictly applies to when Ja'far is speaking to him). Like this, he doesn't have to classify anything, doesn't have to give it a word, and so Sinbad thinks it can stay like that forever—because no matter how Ja'far likes to sort and classify, he doesn't.