There's a lot of things I disagree with Senator Paul about but his essay on getting the police back in their proper role I completely agree with.
The problem is police getting into the mindset where they view their community as an enemy they're occupying instead of neighbors they're helping. Military gear and pseudo-military attitudes are a big part of that problem. I think the worst of it comes from the "War on Drugs." When someone entertaining himself without hurting anyone else becomes a justification for tossing a grenade into a baby's crib the law is causing more damage than the drugs.
Edit: Fixed 2nd link