The clearest set of revelations about Severus Snape’s motives comes from the Pensieve sequence in 'Harry Potter and the Deathly Hallows', but there are important hints earlier in '
harry potter and the order of the phoenix'.
In 'Order of the Phoenix' Harry glimpses some of Snape’s school memories during Occlumency lessons — the bullying by
james and Sirius, the tense, private moments with Lily Evans, and the general loneliness that shaped him. Those scenes plant the seeds: humiliation,
Envy, and a fragile, intense friendship with Lily. Then in 'Deathly Hallows' the floodgates open. The memories Dumbledore asked Snape to store show Snape as a boy, his early friendship with Lily, his brief turn to the Death Eaters, and the fateful night when he tells Voldemort about the prophecy. Most crucially, there’s the memory of Snape begging Dumbledore to save Lily, and the devastating moment of his grief afterward. The memory of his Patronus — a doe — and the conversation where Dumbledore convinces him to protect Harry reveal why he stayed: love, guilt, and a promise.
Putting those memories together makes Snape’s motives painfully clear to me: a mixture of remorse, obsessive love for Lily, a desire for redemption, and a strict loyalty born from that grief. Knowing that changes how I watch every small kindness and cruelty he shows throughout the series.