Simple answer.....then again, not so simple.
You don't know what the REAL dynamics of that relationship are. My guess is she doesn't love him, or she loves him and is angry with him (for whatever reason). The anger may be a build up over something they are clearly not communicating about, or any other multitude of reasons.
My guess is, she doesn't truly love him. So many people enter marriages and relationships for many different reasons: security, challenge, money, companionship, status, intelligence, and guess which one comes last? ....LOVE. Why? Because real love is scary. Why do you think that divorce has almost reached 50% (in my country at least), and guess what? Over 60% of the remaining couples CHEAT! That doesn't leave too many love birds in the nest.
Having said this, people don't marry for love just because of fear. Many people haven't, or won't meet their true soul mates: love at first sight, the feeling of "OMG, I can't live without you" feeling. Forgot which psychologist said the following but it's so true. Don't find a partner with whom you can live, rather, find a partner with whom you CAN'T LIVE WITHOUT! Then you'll know you've hit the jackpot.