When the reader is introduced to Kit, she is on the island of Barbados. She is from an aristocratic family. She's used to having servants and beautiful dresses and all of her needs met. She is not used to doing any work of her own.
When Kit arrives in Connecticut to live with her Puritan aunt and uncle, her world is flipped completely on its head. She is forced to wear plain clothes, she works in the fields, or has to do housework. She puts in long, hard hours every day.
William Ashby is the son of a very wealthy man; therefore, William Ashby is a potentially rich future husband. He has the ability to return her to her previous life style. She would no longer have to work the hard Puritan life. She would have a big house and servants again. Kit could return to being an aristocrat. There's no single cause that makes Kit seriously consider William as a future husband. It's his ability to return her to her previous societal status level that Kit finds so appealing.