Replying to Posts/Comments

As I understand the desire to keep threads consistent as to topic and content it seems to me the subject line of each comment should be the same as the original post?  Yet, when we get the form to submit a comment the subject line is blank.  I usually try to type in the same subject line but sometimes it is long and I forget what it was in the first place.  It would be nice if that field were automatic, any chance of that?