I've asked myself that question, several times before. And to answer it, I always reverse it and look at it from another direction. What compels ME to respond to a thread, instead of simply viewing it?
How often do *I* view threads and NOT respond? The answer to the second question is--quite a bit.
The answer to the first question is more complicated.
It depends on my mood; maybe I'm just in the mood to browse, not type.
It depends on the time I have; maybe a response comes to mind, but it's more than just "Nice!" and so I decide I don't have time to type all that out (as was the case the first TWO times I viewed this thread).
For a photo thread, it depends on the photo. If I just hate it and think "wow, that's just really bad," I'm typically going to just leave instead of saying "That's a terrible photo. IMO." But most of them don't provoke that response for me, so why don't I post a reply?
At least for me, the following things tend to increase the chance that I'll respond to a photo thread:
--There is something in particular I notice that I believe could improve the photo.
--It is absolutely stunning picture that grabs my attention.
--The OP asks one or several pointed questions to invite response (i.e., what do you think of the composition? Is this a little underexposed? What could I do the next time about "x"?)
--There are OTHER responses to the thread and they say something that I want to respond to, or add to.
The photo threads that I am the LEAST likely to comment on are those where I see absolutely nothing WRONG with the photo, nothing I would necessarily do differently, but it just doesn't have a "wow" factor.
I can't really critique it, as I have no particular critique to give for it, but I also wouldn't add it to a "favorites" folder if we had such a thing. It's just a "that's nice enough" photo.
Finally, I (often mistakenly) usually assume that my reply doesn't really matter, because others will reply too.
Or, it could be as simple as the fact that it's posted on a Thursday. Personally, I could just never quite get the hang of Thursdays.