There was a time where you could read Facebook messages and not have to bother replying back because the sender didn't know if you had read them or not. However Facebook introduced the "seen" feature which tells the sender what time their message was read by the receiver. Because of this you now feel that you have to reply so you don't come across as rude or ignoring the sender.
There is an extension which now lets you disable that feature. So next time you read a message the sender won't know if you've read the message or not.