It's a great question. Law enforcement officers can be trained, but experience is the best teacher, and so to maintain a 24 hour presence that can both deter and respond to crime, you need a large number of experienced personnel.
Considering they can get shot at during routine traffic stops, law enforcement officers don't get paid nearly enough, and the job is high stress. So one of the challenges is keeping them on the job and in the field. They have families, and divorce rates on the force are higher than in the general population.
So ask yourself if you really want to go into that field, and how doing that job well will affect your personal life. It might affect when you choose to get married or have children, and you will definitely have to find the right person. I guess that's why so many cops marry other cops.