Some people have theorized that the reason Nina shows her belly off so much is that she has trouble finding shirts that fit. It’s never been brought up in the story but that’s not the reason. She’s willfully breaking the rules because she likes open clothing AND her tummy is pretty spectacular. She has sense enough to know when not to do this, but it is still a risk which she gets a thrill from. I’ve always thought of it as a holdover from her days as a very vain cheerleader. In spite of how wise she seems sometimes she still has a streak of immaturity. Part of the reason she hasn’t had anyone in so long is that vain refusal to settle for anyone but who she likes best. She still has a tendency to value the superficial in a relationship. Her complex relationship with Thomas probably doesn’t help anything either. There was likely some hesitation to let go of that chance with each other that held them both back at one point or another. Use it or lose it, as they say. Sometimes you need to make a decision before life makes it for you.