No, I don’t think that’s it. In chivalry, devotion to your liege lord is a sign of virtue; in chivalric romance, devotion to your love is likewise. Brienne’s case is rather efficient in that the two are one.
I think GRRM made Brienne devoted to Renly was to set up the idea that Brienne’s purpose in life is to protect others to the point of laying down her life in the approved fashion (”all his other knights wanted things of him, castles or honors or riches, but all that Brienne wanted was to die for him”), but that Renly wasn’t actually worthy of her service. Hence why Brienne spends all of AFFC looking for a maiden in need of defending, while slowly forgetting Renly and developing a deeper connection to Jaime.
Now, I have a darker guess about what Brienne’s endgame is than some people would like, but I think it’s definitely grounded in the rules and forms of chivalric romance and thus in Brienne’s character…
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