How are Opal and Winn-Dixie alike?

Opal and Winn-Dixie are alike because they are both sort
of "ugly ducklings" who feel like they have been abandoned and need someone to care
about them.

When Opal meets Winn-Dixie, she is new in
town.  She and her dad have just moved to Naomi, Florida.  She and her dad have been
abandoned by her mom and she clearly feels very alone and vulnerable because of that. 
Winn-Dixie is (allowing for the fact that he's a dog) just the same way.  He is a stray
who has gotten himself into trouble in the store.  Opal, of course, takes him home with
her.

As the book goes on, we see how the two of them are
similar.  To see this, look at the end of Chapter 4 and the very start of Chapter 5.  We
see the author juxtapose the emotions of the two characters very nicely.  Chapter 4 ends
with Opal thinking about her mother and how, if her mother ever came back, she
would

grab her
and hold on to her tight and not let her get away from me
again.

The very next line,
the first of Chapter 5 is

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Winn-Dixie couldn't stand to be left alone; we
found that out real
quick.

Just in those two
lines, the similarity between these two is made clear.  They both have been abandoned
and neither of them wants to be left alone if they can help
it.

It is because they are so similar and need each other
so much that they come to be so important to one another as the book goes
on.