“Must be a loner” is how one reacts when one sees someone (particularly a woman) sitting alone in a restaurant. You don’t have to pity her. She is enjoying her own company and that’s why she is there.
As Charles Lamb said, “I particularly savour the peacefulness of eating alone. Some of my best meals have been with just myself.”
For a lone diner, it is relatively easy to get a place even on a crowded day in a popular restaurant.