There is most definitely not any particular sort of logic to why individuals become what they do here—or at least insofar as Aziraphale has been able to find. He's asked around plenty and attempted to make some judgement calls based on the other beastfolk that he knows, but he still comes up short.

Nevertheless, it is a thought that he likes to entertain. That there must be some grand underlying reason behind it.

He worms his face closer, trying to find his way to Crowley's lap like some oversized housecat.

"Naga would far too unimaginative," he remarks, almost haughty about it.

"I would think, if there were any process behind it, that is, that it would speak to a person's nature."

That and he thinks that the bright, colourful wings would be an entirely enjoyable sight on Crowley.