start by expressing gratitude for still being under consideration for decision and mention that you are writing to reaffirm that pitt is your top choice. Then I wrote a little bit about why I started to like the school to begin with, and then how visiting/interviewing there made me even more confident in the school because of xyz. They want to make sure that who they let in at this point is truly passionate about the school so you can really personalize this part. Mention qualities about yourself, with examples, that show why you would be a good fit for them and how you'll accentuate their class. At the end you can mention some updates on your application, or current classes youre taking, or just anything you've continued doing since submitting. End with another statement summarizing your interest, and your appreciation for being considered.