It's nice to be sitting with Trowa, but still slightly distracting.

(Why do you think that might be, idiot? it's not like you just started dating your closest friend who you've been attracted to for years, or anything like that.)

He applies slight pressure to the tip of his tongue with his upper teeth as he tries to concentrate on the book well enough to not distract Trowa further from his book.

It's not that hard, if you dissociate enough. Which he doesn't want to do, so it's quite a bit harder.