To me, the reason that Gurathin and MB get on each other's nerves is that they are so very alike. MB is cheezed off that it was found out, and the cherry on top is that it wasn't somebody like Ratthi, but Gurathin whose skillset and personality overlaps with MBs in several ways.

Yes, this is my read on it as well - showverse, anyway, I don't remember the books well enough to say! (But the writer is apparently involved with the show and pleased with the adaptation, so presumably the show interpretation is also what's intended in the books.) One of the Tumblr posts I saw described it as two adopted cats angrily reacting to the other cat's adoption, and I feel like that's so true. XD