It's all from context. Duo is incorrect not to accept "the reading" here. It's correct that in most circumstances, adding "the" would make no sense for this sentence, but lacking context it should accept all possible correct interpretations, including a previously-specified reading being the person's passion.
In English, we don't tend to use articles with concepts and generalities ("I like potatoes" rather than "I like the potatoes", which in English would be about some specific potatoes). In French, these generalities always get definite articles (ie, those two potato sentences are identical to each other in French).
"Grand" is an exception. See here: http://french.about.com/od/grammar/a/adjectives_4.htm
Hi Katherine. I see what you mean, but I would have to respectively disagree. I think a phrase like, "She was his great love." are used. Such phrases imply not only that she was his greatest love in a comparative sense, but also that it was a love on an epic, romantic, grand scale in his life. It has the same meaning with regard to other events.