Original: In Chapter 33 ("The Prince's Tale") Snape's memories reveal that he loved Harry's mother, Lily Evans, but their friendship ended over his association with future Death Eaters and his "blood purity" beliefs.
Simplified: In Chapter 33 ("The Prince's Tale") Snape's memories show that he loved Harry's mother Lily Evans, but their friendship ended over his association with future Death Eaters and "blood purity" beliefs.