Before Thad, Dagny had never really gone in for overly romantic sentiments. Calling your boyfriend your 'other half' just seemed like saccharine nonsense. Her worth wasn't reliant on someone else. She didn't need a man to make her complete.
But this man? This man made her feel like together they were more than the sum of their parts. Perhaps that was what 'other half' really meant. Dagny melted into the kiss, more than okay with that.