One, you could become so jaded about the job that you lose your focus. You forget your true job description. A police officer is a "human service provider." You provide service to those in need. Even making an arrest is a public service. You have made it safer for the public by removing this perp.
Don't become so jaded that you forget to be a good public servant. Treat everyone as you would want your mother to be treated: with dignity. The day you treat everyone as a perp, please do the professionals a favor and turn in your shield. No cop ever stands so tall as when he or she reaches down to help those in need.
The next flaw could be complacency: "Nothing ever happens in this city, nothing happens in my precinct, and so what." With this attitude, your tactics are lowered, your skills diminish, and you are there only to wear a uniform and make your check.
Let me remind you that the school tragedies we've had in this nation were not in the big cities but small, quiet communities. Many of our major headlines come from the least suspecting places. Luckily, the officers involved have remembered their callings of providing great human service to save some kids from a creep. They kept their warrior mindset there and waiting; they served and did the job!
The key point here is to keep your edge and keep your focus. The "job" is not a job; it is a true vocational calling that few can perform. You must stay on top of your game so that one day when the call does come out you will be prepared and perform brilliantly.