When Opal first meets Winn-Dixie in the grocery store, she sees the dog smile—something she has never seen a dog do before. When the grocery store manager begins to cry, Winn-Dixie licks him. When Opal manages to get Winn-Dixie outside, she sees that the dog is a mess. He has many bald spots, and he is so skinny that Opal can see his ribs. The dog pulls back his teeth and smiles, as if to say that he knows he looks like a mess, and Opal likes Winn-Dixie's sense of humor. Opal also appreciates that like her, the dog does not seem to have any family. While she does have a father, her mother left her when she was three years old. Also like her, the dog appears to have no friends. Winn-Dixie appears to listen to Opal when she speaks to him about her mother and how she is going to ask her father, the preacher, to tell her about her mother. She likes all these qualities in the dog.

In the book "Because of Winn-Dixie," Opal likes the dog Winn-Dixie for a few reasons.  Some of these reasons are immediate and physical, but the most important reason is because of Opal's own personal situation and the way the dog makes her feel.

Physically, Opal likes the way Winn-Dixie "smiles."  And she likes the way he sneezes.

But really, the attraction for Opal seems to be that the dog, like herself, is all alone in the world.  Of course, Opal does have her father, but at the beginning of the book they're kind of distant from one another.  In Winn-Dixie, Opal finds something of a kindred spirit and she and he can rely on each other for comfort and a sense of belonging.

Eventually, of course, Winn-Dixie helps Opal solve her problems of loneliness on a human level -- helping her make friends and connect more to her father.