After Alex witnesses Riggs and Meredith's flirtation in action, he asks Mer why she isn't with him, since he can tell she wants to be. Meredith says she doesn't want to be with Nathan, but Alex sees right through her denials. Watch this sneak peek from Season 13, Episode 15, 'Civil War,' and don't miss Grey's Anatomy THURSDAYS 8|7c on ABC.