Robert Wachter, writing on Medium:

Why is masking so difficult to maintain among the public? In Asia, face masks are now seen as a normal accessory. In the U.S., they’re still seen as awkward and stigmatizing. Historically, they have been a sign of illness or danger. This aversion, plus the fact that the benefit of masks mostly accrues to others, is why we need to make mask-wearing mandatory as long as SARS-CoV-2 is active in our communities, at least in closed spaces (as San Francisco has done).

One of the most common questions is whether it is necessary to wear a mask when walking or exercising outside. Empiric and simulation studies have shown that there is practically zero risk of viral spread when one is outdoors and keeping a distance of greater than six feet from others. I personally don’t wear a mask when walking the dog (but I do keep one with me just in case I encounter someone at close range). But I always wear a mask inside, or if an encounter within six feet is likely.