Mary was thinking the exact same way that her son was. How much time did they truly have together before this place or whatever it was sent one of them back? Or sent Ben back? There was no way of knowing; there was no way of predicting any of it. Who knew if they would ever have this chance again? Being Winchesters, probably not.
Mary also didn't want to pressure Dean either. "I don't want to push you to do something if you don't want to either. It would be a lot for him and if his mom didn't want him to know...but we just don't know how much time we have left either. This place is unpredictable." Mary squeezed Dean's arm. "I want you to have the chance to know him as much as I want the chance for myself."