how could unrequited love be the theme of great expectations?

This is mostly between Pip and Estella. It would seem Estella didn't share Pip's enthusiasm for a relationship. There were of course all types of mitigating factors like Miss. H. THere was also the sense that Pip should have been more open to Biddy's affections.

The theme of unrequited love begins with Miss. H. being left at the alter, and her desire to set up another generation of the same. The emotions and feelings that stayed with Miss. H thoughout her life, were the reason she decided to take her revenge on "men" by turning Estella into a woman who couldn't return love. Pip, by loving Estella, is Miss. H's vengeance; there's unrequited love all around.

Dolge Orlick loves biddy but biddy does not return the love so we can call this unrequited love