ON the other hand, Elizabeth was very pleased with Bingley's behaviour to her sister Jane. Why? How did Bingley behave?

Bingley pays a lot of  attention to Jane, and Elizabeth concludes that Jane is falling for him but hiding it well. Much like his Jane, Charles Bingley is an amiable and good-tempered person. He seems to connect with Jane on a socially equal  level even though he is much wealthier than the Bennets. Elizabeth likes this about Bingley.