Why did John Muir believe Hetch Hetchy should not be dammed?

The simplest answer to this question is that John Muir is a preservationist. He believes that the land should be left alone in its natural beauty. He's not completely against civilization utilizing land to build cities and infrastructure, but he is against this when it destroys some of nature's most impressive and beautiful creations in the process. Hetch Hetchy is one such area. He wrote the following about Hetch Hetchy:  

Hetch Hetchy Valley, far from being a plain, common, rock-bound meadow, as many who have not seen it seem to suppose, is a grand landscape garden, one of Nature’s rarest and most precious mountain temples. 

Part of his argument against the dam is that the dam would be intruding on a federally protected National Park. That park is Yosemite National Park. John Muir is adamant that Hetch Hetchy remain untouched because it has already been declared part of something that will remain natural and free. He is afraid that if this dam is built, other parks and areas of the country will also be in jeopardy.  
John Muir also believed that the Hetch Hetchy dam plan was really nothing more than a money-grabbing opportunity by a few commercialization proponents looking to make a lot of money quickly. Muir simply doesn't believe that something as beautiful as Yosemite Park should suffer under the grip of commercialization.  

This grossly destructive commercial scheme has long been planned and urged (though water as pure and abundant can be got from sources outside of the people’s park, in a dozen different places), because of the comparative cheapness of the dam and of the territory which it is sought to divert from the great uses to which it was dedicated in the Act of 1890 establishing the Yosemite National Park. . . .
These temple destroyers, devotees of ravaging commercialism, seem to have a perfect contempt for Nature, and, instead of lifting their eyes to the God of the mountains, lift them to the Almighty Dollar.