Toby works in HR. Michael hates no one more than Toby. 
Toby did leave Dunder Mifflin after he put his hand on
Pam's leg one night (as he was in love with her). He went to Costa Rica, broke his neck, was laid up in hospital for 6 weeks, and then returned
to Dunder Mifflin. He worked there for a week, before Michael realized he had returned. Michael's scream of horror was heard at Corporate... suffice to
say, Toby allowing Michael to walk all over him is the reason that Michael treats him the way he does.