To be a cop, you have to be a dick
Or at least the overwhelming majority of the time. There's no denying that police officers do a necessary service, but I would still insist that in order to routinely have the type of unpleasant confrontations police have with private citizens on a regular basis, you'd have to be a bit of a dick or worse. Even though these enforcements of law are often justified, as a cop, you are going to constantly be put in situations where YOU are the individual enacting the legal process that has the direct result of ruining someone else's life.
A person that doesn't have trouble punishing and bringing negativity to people for a living has to be callous. Why should we disregard the callousness of an individual just because of the possibility that their actions in this position are partially motivated by a dedication to justice?