Luca had assured Sophia that he wasn't different when he was in the office compared to when he was at home, but as he became openly possessive over her, she wasn't so sure.
The kisses at home were simply because he felt like it. The one in his office was full of so much feeling. She could tell by the way he was holding onto her that he was resisting the urge to squeeze her tightly to reassure himself that she was in fact within his grasp.
When it came to other alphas, he seemed to always be like that. That seemed to be the hardest instinctual urge for him to ignore. However, it wasn't one she didn't mind.
As he kissed her, clearly frustrated at the thought of alphas trying to influence her with their pheromones, it made even less sense why he was hesitant to bond with her.
If he allowed her to give herself to him completely, it would take care of the problem instantly.