Comment by arcsin

One reason is that comments get stale. People need to maintain them but probably won't. Second reason is that they think the code should be self-documenting. If it's not then you just need better names and code structure. Many books like clean code advocate this approach, and that's where I first learned the idea of don't write comments as well.

Personally now I've held both sides of the argument at different times. I think in the end it's a trade-off. There's no hard and fast rule, you need to use your best judgement about what's going to be easiest for the reader and also for the people maintaining the code. Usually I just try to strike a balance that I think my coworkers won't complain about. The other thing I've realized that makes this tricky is that people will almost always overestimate their (or others) commitment to maintaining comments, and/or overestimate how "self-documenting" their code is.