I think some people dislike the police because:
They don't agree with a particular law (forgetting that the Police don't make the law).
They don't think the law applies to them, also known as the "why aren't you out there catching real criminals" syndrome.
They've fallen for Paper's "the police are brutes" or "the police are useless" slant on events (enough threads/posts on here make me suspect that many people take what is reported at face value).
They expect the Police to fully investigate every single crime fully, with full CSI support...
They mistake a certain level of cynicism/caution in the way the police may treat people. with rudeness - not realising that while their insurance documents are indeed at home, and it is indeed a mistake with the computer showing the car as uninsured and unable to remember the insurance company, but they've also had that line hundreds of times (sometimes from quite respectable looking people) who aren't insured. Or that whilst you're not a threat to them, they've met enough people who also looked harmless in similar situations, but did turn violent.
They mistake the legal requirement for the police to investigate certain complaints (even if they seem silly) impartially.
They don't have a clue about the role of the police, verses the roles of the CPS, Magistrates/Judge/Jury, thus blame the police when someone is charged or convicted (or let off without charge/a short sentence).
And not forgetting the all time favourite reason to hate the police: Because it's "cool".
I don't necessarily like individual officers, but I wholeheartedly respect the police force as a whole, especially the guys on the ground who are basically doing a job that no one who is fully sane

would want to do for the wage that they are paid.
I've heard comments from some officers that they would much rather deal with a "career" or "repeat" offender, than many of the "occasional" criminals who have a good job/background and whose only offence might be something like getting completely drunk then try to drive...