With comments enabled, people have three options after they finish reading a post, looking at pictures, or watching a clip.
1. Go Elsewhere (either somewhere else on the site or to a different site)
2. Make A Purchase
3. Leave A Comment
If comments are disabled, they only have two options.
Also, the act of leaving a comment gives people the illusion that they've accomplished something, making them even less likely to make a purchase.
Unless you're trying to establish an online community of sorts, which is a difficult thing to do when revolved around a product largely consumed in solitude, the typical commenter is a essentially a virtual loiterer.
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