I see two possibilities: 1) he went on several perilous quests specifically for her approval. -or- 2) He asked for her blessing to Marry Annabeth. She refused, and then he pretended to sneeze. Athena said, Bless You, Percy said thanks, and Athena facepalmed
They’re going to get married at Long Island Sound, invite all the campers, Chiron, the gods, their family members, etc, and then go to Greece for their honeymoon