Comments have to maintained, too. And quite often, they aren't. The result is a program that lies to the reader. The greater the proportion of comment lines, the greater the risk that someone downstream will break the comments while extending the code. Broken comments adds to downstream risk. Broken comments also add to the risk that the reader won't trust any of the comments, which further burdens maintenance efforts.

This is one of the forces that needs to be balanced in order to find the right mix of comments to code.

The eXtreme Programming folks take an extreme view on comments. They hold that comments are one of theCode Smells that are clues that your code has problems. If you need to write lots of comments, it's an indication that your code isn't clear enough, and that you should put effort into cleaning up the code so that it describes itself.

(This is a point on which reasonable people can differ.)