They had made a promise, a binding agreement between the two. Neither Wendy (who had more boyfriends than Mabel could count on both her fingers and toes) nor Mabel (whose summer romances weren't epic) would ever worry about boys romantically again.
The promise seemed easy, and it turned out that it was. Mabel quit flirting and dreaming of a date like on her eighties cartoons.
After all, she now only ever thought of Wendy, with her awesome jokes and wonderful hair. Mabel finally knew how Dipper had felt.
Mabel kept her promise and secret. That was all she could do.