In many cases, I understand and appreciate what the person is writing (in each sentence), but have little to say in response, even if I really like what the person is saying. So when I have little to say in response, the number of replies I can make is rather limited [ah okay, oh ok, so basically, ah sure, oh interesting, oh true, good point, intriguing, nice, etc...]. Sometimes I try to repeat what they say and then say "that's interesting". But then people have told me that I'm like a void.
Of course, it's not required for me to say anything substantial, but I do want them to write more (and to feel appreciated). And it can be discouraging to them if they can't tell whether or not I truly appreciate what they say or if I'm just saying it just to be nice.
In fact, this frequently happens in academic emails (especially with professors) - although the roles are reversed in this case - I might send paragraphs of replies, and the professor may only send a reply that's a sentence long (even if he appreciates everything I've written). It might actually be easier if we could just comment on emails like we can insert comments in Microsoft Word documents (or in the margins of a final paper) - but that isn't possible with the email software that we usually use