It is awfully easy for one party or the other to get in the habit of just dropping in unannounced. It is easy to drive by and see if you are away from home and that brings the question, "Where were you?" It is easy to drive by and see strange cars and ask, "Were you having a party?" Or how about a phone call that says, "I noticed your yard needs mowing." In other words, it is easy to get into one another's business.
Also if you have a disagreement it is easy for the parents to get involved and that can cause hard feelings, for both parties.