De acordo com tvtropes :
He is a lifelong sufferer of Obsessive-Compulsive Disorder and has many phobias, and those obsessions became crippling after his breakdown, forcing his retirement; he recovers throughout the series, though he is never fully "cured". His OCD is also the reason Monk was such a successful policeman; one of his compulsions is paying amazing attention to details.
Ele se tornar um policial antes de ir para o detetive nunca é discutido.