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The basis for Michael’s disdain lies in his perception of Toby as someone who simply doesn’t care about what Michael values—like being appreciated or feeling accepted. This tension is accentuated by moments such as the scene where the actor enters Michael’s office, jokingly writes a birthday message for Meredith to save time, and senses Michael’s underlying resentment.
According to the actor, Steve Carell's Michael "started to feel this hatred" intentionally, and the writers allowed that feeling to build organically. No formal decision was made to have Michael hate Toby; it just grew from the character interactions on the page and subsequent performances.