The reader is introduced to the character of Sophia Western in Book 4, Chapter 2. Quickly, we realize that she is going to be the story's main heroine and Tom's major love interest. Tom is attracted to Sophia for her talents, beauty, and grace. Sophia takes a similar liking to Tom. The problem is that many others see the same things in Sophia that Tom does. This, plus her family background and wealth, make Sophia a very eligible bachelorette.
Sophia's father, Squire Western, is eager to marry off his daughter to a well-to-do young man, Mr. Blifil. Sophia must frequently find ways to dodge this unwanted marriage prospect. Remember that this story is set in a time and place in which a father had every right and obligation to be his daughter's matchmaker. She would much rather marry Tom, but her father strongly disapproves of this idea due to Tom's lower social ranking.
While Sophia and Tom clearly have affection for each other, their relationship does hit some turbulent spots. Sophia is furious with Tom after he sleeps with Mrs. Upton as well as other women.
They still desire each other throughout the narrative. What changes to finally allow their union is Tom's eventual advancement in society. When Tom becomes Squire Allworthy's heir, Sophia's father finally accepts the young man as marriageable material. They are married the very next day.