Without the pressure of having to react and allowing oneself to just be while observing takes the pressure off "doing" something together. The listener gets to hear their lovers voice, another arousing sense besides watching the fall of hair, the roaming of eyes, action in stillness.
Conversely, the reader is in the spotlight and demands the listeners attention at every word. Even if your performance isn't Emmy award winning you're still with your lover. They appreciate the effort.
So while cuddling up against each on a soft couch to watch TV is romantic in itself, if you want to do something different this one night...then pick a good story and read out loud.