Willaim's presence is indeed like the sun coming out! He showcases a different style of liveliness from that of the Crawfords. It is possible, after all, to be merry in Mansfield Park without having to resort to risqué repartee or questionable behavior.

He inspires and entertains everyone around him. Even Lady Bertram. With such means in his power he had a right to be listened to; and though Mrs. Norris could fidget about the room, and disturb everybody in quest of two needlefuls of thread or a second–hand shirt button, in the midst of her nephew’s account of a shipwreck or an engagement, everybody else was attentive; and even Lady Bertram could not hear of such horrors unmoved, or without sometimes lifting her eyes from her work to say, “Dear me! how disagreeable! I wonder anybody can ever go to sea.” Chap. 24

Mrs. Norris, of course, is a different story, and is clearly jealous that a poor relation of hers, under the patronage of Sir Thomas, should get so much attention and appreciation form others.

He even manages to make Henry feel some regret about his easy life, albiet temporarily. ;-)

I also think that it is very sweet of Sir Thomas to appreciate the effusion of love and spirits displayed by Fanny upon William's arrival. He does not feel that it reflects badly on them that she shows affection so openly for her brother and is more animated than she has presumably ever been at Mansfield Park. ;-)