Chubukov changes his behaviour towards Lomov because, from his experience, he was expecting Lomov to request a loan, a request Chubukov was unwilling to fulfil. No, Chubukow is not at all sincere. But, when Lomov asks for Natalya's hand in marriage, a proposal that catches Chubukov by surprise, his attitude changes. Lomov merging with their family would mean additional wealth and prestige for Chubukov, which also explains his sudden change in behaviour. His references to Lomov as "my darling" and "my treasure" after Lomov's proposal reveals Chubukov's fickle nature when personal interests are concerned.