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If you evolve a Paras into a Parasect, you killed the actual insect part and the mushroom becomes the sentient part of the Pokemon. The white parts of the eyes indicate that it's actually dead, as they've faded away and show no consciousness.
It's even worse when you remember the OG Pokemon anime had an episode where a girl tries to get her Paras to evolve to use it's larger mushroom for medicine and it refuses to evolve.
