I find there's two primary reason cops will pull you over:
1) You're doing something that makes them take notice of you
2) They're trying to meet their quota, and running plates just to see if they'll get a hit.
In my town, the cops are seriously underfunded, so they spend more man hours on violations that are fineable and will make them money, ie. speed traps, than minor crimes, ie. someone breaking into your home and stealing all your posessions. The local police have actually cited a policy to someone I know who's house was broken into and robbed that they will not send someone out to investigate a house that has been robbed unless under certain critera. Meantime state troopers will sit in the bushes for hours waiting to nab motorists going 10 MPH over the speed limit, endangering no-one. Talk about a crime.
The police certainly provide a service to the public, but in the realm of minor violations and white-collar crime, in my mind it is no longer "To protect and to serve." It's now "To project and to earn."