It probably doesn't hurt if Celestia happens to be grading Twilight's letters at the time. Nah, I understand completely. Celestia clearly is still focusing on the work, but they way she's holding Twilight makes it clear that she really loves having the company. Twilight looks like she couldn't feel calmer than being up against her mentor, reveling in the warm feeling of Celestia's coat, the scratching sound of the quill against parchment and the princess' breathing, maybe even sensing her heartbeat.
At any moment Celestia could suddenly rest her jaw on Twilight's head to pet her a little, just for a quick breather between papers.
Another wonderful piece! It's great to have an artist like you supporting this pairing! You seem to have drawn Twilight a little bigger than normal too, which I like. I dunno, I guess it's because it makes Celestia look legitimately bigger rather than Twilight simply appearing to be a foal in comparison.