Character Analysis
The Sultan of Syria is a man who's accustomed to getting his way. He also appears prone to obsession. When he hears about the reputation of the beautiful Custance, he finds himself unable to stop thinking of her, so that "all his lust and al his busy cure / Was for to love hir" (188-189). Basically, he's got a crush, and he's got it bad.
Gotta Get the Girl
The Sultan isn't the first medieval character to fall in love with someone he's never met merely through knowledge of the woman's reputation. Still, his refusal to listen to his advisers' well-intentioned warnings about the "diversitee / Bitwene hir bothe lawes" (221-222) marks him as particularly hard-headed. We have to question the prudence of someone who would abandon his birth faith, and force his friends to do the same, out of desire for a woman he's never met.
Here's a theory: we might read the failure of the Sultan's conversion as evidence that he did it for the wrong reasons. Unlike other characters converted by Custance, the Sultan switches teams for selfish reasons. This is not okay in the world of the tale, and accordingly, the Sultan meets a sorry end. With him, then, "The Man of Law's Tale" is raising questions about the proper and improper reasons to undergo the Christian conversion.