Once upon a time, Jenna was a teenager too, and once upon a time it as her and Miranda and Grayson and John in some weird love rectangle. Of course, it wasn't really like that, but that was how Jenna had seen it at the time.
Miranda had liked Grayson and so had Jenna, but Grayson only had eyes for Miranda, so Jenna had settled for John who had had eyes for a girl named Isobel.
It was not love between them, Jenna told herself on countless occasions; she just liked to pretend it was. It was easier to be in public with him and think of it as love than to allow the reality to enter her mind that they were simply using each other to cancel out the pain of those they could not have. Or, at least, that's what she said when she was alone to justify her own behavior.
And the I love you's that were spoken while they were in the throes of ecstasy—Jenna wrote them off as anything but the truth. Because Jenna could not—ever—be in love with John Gilbert.