Once they meet, Winn-Dixie and Opal are inseparable. He goes everywhere with her: to the library, work, and even to church. Can't get much more companionable than that. And Winn-Dixie brings out the best in Opal, hands down.
Both are "almost like orphans" because Opal can't reach her father and can't remember her mother (3.2). Ditto for the dog.
Winn-Dixie is friendly and outgoing and tries to bring out the best in people. Ditto for Opal. And being with Winn-Dixie gives Opal the courage she needs take action.
Apart, Opal and Winn-Dixie are lonely and sometimes misunderstood, and in Opal's case, shy. Together, they are a Dynamic Duo (Woohoo! Alliteration!).