Curate, connect, and discover
I headcanon regulus as having really bad anxiety and I think that he doesn't feel that as bad or get weighed down by it as much when he's in his animagus form or when he's with Lily or James.
Just imagine James sitting in the common room with Lily cuddled into his side, reading a book while he talks to his friends. Then imagine a black cat sleeping in lily's lap, possibly hidden beneath a cover.
Imagine lily walking about hogwarts with regulus sleeping in her book bag. Now and then she puts her hand in the bag to remind him she's there.
Or regulus, in his cat form, sneaking into the Gryffindor Tower at night to snuggle with James without setting sirius off.