Wolfgang missed his mother and Nannerl. But his sister was a wife and mother now. She had no time for Wolfgang's tomfoolery. His mother had died in 1778, when he was still clouded with visions of love with Constanze's sister. They traveled together and when she fell ill (after having given up her entire life to his success) Wolfgang sank into depression. Now, his heart was burning for love and acceptance again and his father rejected it. Heartless man! "This is why I didn't want to tell him until now...." he'd kept him in the dark on the seriousness of the relationship, and the same was probably true to the Weber's as well.
As soon as Stanzi voiced her idea he lit up, hugging her cheeks within his hands with a kiss. It was improper indeed! Wolfgang had never followed rules and regulations, he felt stunted by them. They had shared little kisses here and there and probably acted far too intimate for a non-married couple in their day in age. He was in love and he felt love from her. It was all Wolfgang ever wanted.