Walking a dog is a top responsibility of dog ownership.  Keeping your dog on a leash during walks is part of that responsibility. It keeps your four legged friend safe and demonstrates that respect your neighbors.

Check you lease. The pet addendum to your lease states that dogs must be kept on a leash at all times.  This a quite common in apartment communities and for good reason.

However, the risk of a lease violation should not be the primary reason for keeping your dog on a leash.  You should do it because it is the right thing to do and the safest for you, your dog and others.

You might hear some people say, "It's okay! My dog is friendly."  That may be true but you don't necessarily know about other dogs in the neighborhood.  Other dogs may be nervous, reactive, fearful.  A fearful or nervous dog may react in unpredictable ways.  He may lash out and bite your dog and cause serious injury or worse.

The other dog may be in training. If the dog is new to the family, they may be working with him or her to properly walk on a leash, demonstrate good 
doggy" etiquette and social skills.  If your dog is allowed to run up on them, this process may be interrupted and cause issues for the dog and its family.

It may be as simple as the other dog is owned by people who want to be left alone.  

So you see, everybody wins when you keep your dog on a leash.