Each patronus links to someone's personal life. Harry's patronus is a stag because his father was a stag animagus. Snape's patronus is a doe because he loved Lily and her patronus was a doe. you patronus can also link to your personality. In the books, Tonks patronus changed because she was no longer her normal self after she was sad about LupinSirius.
It was not Lupin she was sad about, it was Sirius