Strange. I don't like it when people write "thank you" emails. They have a question, I reply - end of story if the problem was solved. I hate it when I open my ticket system and I have to spend the first five minutes of the day reading through tickets that were already resolved and setting the ticket status back to "resolved" just because some people felt the urge to write a thank you email and by doing so reopened the ticket.
We also aks customer for reviews after they order and while it is nice to get positive feedback I always think: "If you liked our services so much why did you not click the link to the review website in the email and tell the world, instead of clogging up my ticket system with a thank you email. Luckily most do, though.
If you think you provide a good service and you are craving for customer feedback, then simpy ask for it. Put a link to a review website in your email signatur that says:
"Liked our service? Don't tell us - tell the world!"
Actually from my experience I know that the majority of people like to reward good customer service - if you provide an easy channel to say "thank you". And let's face it: While a personal thank you email may provide a warm fuzzy feeling, a five star rating on a review website is worth more in the long run.
The reason most customers don't write back thank you emails in my opinion is that in most companies the person that answered their email is not necesarilly the person who will receive the "thank you" mail.