You showed the diary to Hermione previously, but after a couple dozen of attempts to reveal hidden text, she concluded that the diary is, simply, empty. That just did not seem right to you. Why would this old diary, that looked so worn in, be empty? Why is it here at all?
As Ron and Hermione were still at the common room working on a Defence Against the Dark Arts essay you had a chance to finish earlier, you take it out to have another look. You can feel an odd pull towards the item that you can't quite explain.
You turn the diary in your hands a couple of times and without thinking about it too much, you grab your quill and start writing.
" The ink shone brightly on the paper for a second and then, as though it was being sucked into the page, vanished. Excited, Harry loaded up his quill a second time and wrote, 'My name is Harry Potter.'
The words shone momentarily on the page and they too sank without trace. Then, at last, something happened.
Oozing back out of the page, in his very own ink, came words Harry had never written.
'Hello, Harry Potter. My name is Tom Riddle. How did you come by my diary?' "
When asked if Tom Riddle knows anything about the Chamber of Secrets, he says yes, and offers to show you.
You think of all the people that have been attacked (poor Colin Creevey was now one of them too, although it is kind of refreshing not having him following you) and you just know you have to see what this Tom Riddle knows - whatever it takes to get to the bottom of this!