From the essay “Dealing with a stray dog” by Akira Hasegawa comes a first-hand account, by Daido Moriyama himself, of how this photo was taken:
The photograph first appeared as a single image within the series Nanika he no Tabi (En Route to Something) in the March 1971 issue of Asahi Camera. As Moriyama himself recalls, “right after New Year 1971, I took a picture of a stray dog in Misawa up north in Aomori where there was a US military base. I was heading out in the morning with my camera in hand and took one step out of the hotel, when right there in front of me was this stray dog wandering around sunning himself. Just like that I pointed my lens at the stray dog and clicked the shutter a few times; later on, his moment in the light was printed as a full spread for a photo magazine series I was doing at the time”. As simple as that. Yet, now that same photograph “catches the eye of photography fans and somehow remains a favourite in Japan and around the world. It’s become a print that wanders around between museums and galleries and private collectors” (from an Asahi Shimbun newspaper essay by Moriyama). A turn of events the young photographer could never have imagined. (the essay “Dealing with a stray dog” is published in the book The World through My Eyes by Daido Moriyama, Milano: Skira, 2010, p. 17)
In 1999, fellow photographer Leo Rubinfien wrote an exhaustive essay on Moriyama’s work for the Art in America magazine. What’s especially interesting about this essay is that it provides us with a good explanation of the symbolic of the “stray dog” both for postwar Japanese culture and for Daido Moriyama (in an argument divergent from Kazuo Nishii’s comment quoted above):
Since the Second World War, the image of the stray dog has wandered into Japan’s best art often enough to have us ask what, in that famously rule-bound, rank-conscious land, such a pariah might mean. As nearly as I can tell, its earliest appearance was in Akira Kurosawa’s 1949 film Stray Dog, where it was not a character but the metaphorical name for a young, murderous pickpocket, demobilized from the Emperor’s army into the bomb-blasted city with no home to return to. At the start of the chase, the stern senior detective warns that such mined men am stray dogs, to be put down before they turn into mad dogs, but his despondent acolyte pleads for compassion, recalling that in the chaos of 1945 he might easily have become such a dog himself. The stray is there again in Susumu Hani’s exquisite She and He (1960), this time as the companion of a pathetic ragpicker who is one of the two principals of the story. The dog is pretty much this outcast’s alter ego, and when at the film’s denouement it is hideously tortured by the children of a cell-block town of materialistic salary-men, the man suffers equally, and we with him. […]
For Moriyama to identify himself with these beasts is remarkable. The West maintains a pantheon of alienated heroes, and in its romantic modernist tradition, the bohemian, rebel, tramp or hollow-hearted etranger have been thought bearers of authenticity and moral legitimacy. But in Japan an outsider is truly an outsider. The hero-outcasts of its premodern folklore, the dispossessed lord Yoshitsune, for example, or the 47 vengeful ronin, are not so much opponents of society as plaintiffs for a justice that society has refused but could easily give. The true renegade–with no home village, no pedigree, no uncles or cousins to protect him, no company, guild, obligations, diploma or calling card–is suspicious even to the most free-thinking Japanese. In the less liberal he provokes revulsion and anger. (“Daido Moriyama: Investigations of a Dog” by Leo Rubinfien, originally published in Art in America, October, 1999)
In Phaidon’s monograph simply titled Daido Moriyama, this photo is reproduced on page 55. The author, Kazuo Nishii, proposes the following explanation on the opposite page:
Two versions of this picture exist, printed with the dog facing in opposite directions. Moriyama went to Misawa in New Year 1971 and observred that it had much in common with many base towns, with its øbarbers, cabarets, boutiques, beauticians and oculits… all lined up” and “dogs everywhere”. Urban dogs were often featured in postwar European photography, fighting and snarling, symbolizing animality. Moriyama’s dog, on the other hand, seems to have been taken from a kindred dog’s-eye point of view, as if merely encountered rather than elevated into a symbolic order. (New York: Phaidon,  2012, p. 54).
About Daido Moriyama:
Born in Ikeda, Osaka, Daidō Moriyama studied photography under Takeji Iwamiya before moving to Tokyo in 1961 to work as an assistant to Eikoh Hosoe. He produced a collection of photographs, Nippon gekijō shashinchō, which showed the darker sides of urban life and the less-seen parts of cities. In them, he attempted to show how life in certain areas was being left behind the other industrialised parts. Though not exclusively, Moriyama predominantly takes high contrast, grainy, black and white photographs within the Shinjuku area of Tokyo, often shot from odd angles.
Moriyama’s photography has been influenced by Seiryū Inoue, Shōmei Tōmatsu, William Klein, Andy Warhol, Eikoh Hosoe, the Japanese writer Yukio Mishima and Jack Kerouac’s On the Road. Moriyama has written a memoir titled Memories of a Dog. (Wikipedia)
Below are some more resources about Daido Moriyama (this section is likely to be updated from time to time):
The International Center of Photography Library blog: “The Daido Moriyama Photobook Collection at the ICP Library” by Russet Lederman, May 14, 2012.
I remember my husband’s purchase of our first Daido Moriyama photobook. In fact, I remember it very well because it was also the very first Japanese photobook we ever bought. At the time, 10 years ago now, neither of us knew that this seemingly innocent purchase would signal the beginning of what we now fondly refer to as our Japanese photobook acquisition disorder. In 2002, he attended a gallery opening for `71-NY: Photographs by Daido Moriyama, an exhibition that took the then risky stance of presenting both photographs and photobooks as equals within a gallery installation.
Michael Hoppen Gallery has numerous photos from various series, including the Shomei Tomatsu series and the Thights and Lips series.
Steven Kasher Gallery has 94 reproduction of Moriyama’s photos shown in medium and large formats alongside detailed information. From March 28th through May 4th, 2013, the Steven Kasher Gallery is hosting what it claims to be “the largest and most comprehensive exhibition of Moriyama’s work ever mounted in an American art gallery”: Daido Moriyama: Now and Now
“Daido Moriyama: Stray Dog of Tokyo” is a documentary produced in 2001 by Viz Pictures  for its New People Artist series. Check its official page , read a review of the DVD over at Blog Critic, buy it on Amazon or rent it for $2,99 (48 hours) on Filmbinder. Watch the trailer bellow: