He realized he had actual romantic feelings for her when Sonia is between 15/16! He's extremely alarmed by this, and feels incredibly guilty! He's always found Sonia cute and cared for her of course since she's his best friend's daughter, so when he begins to see her as more than that he starts to really internally fight with himself over his feelings for her. Especially after his divorce from his wife (which leads into an ugly custody battle for his son, Nikolai). He becomes lonely and desperately wants to give into Sonia's feelings (she's been saying she wants to marry him since she was a kid..... and she meant it LOL ;;;;). But overall its a really, really rough time for him internally and emotionally.

This results in him going out of his way to avoid her for a time out of guilt and trying to contain his own feelings, and Sonia does not take it well at all!