They kiss. It means nothing.
Juliet doesn't love Jack. She feels affection for him; she likes having him around. He's a good leader, a good friend.
A good man.
But she is not attracted to him.
She wonders why he kisses her. Maybe because he likes the feel of someone in his arms, the way she does. Because as much as she doesn't love Jack, she doesn't object to the kiss, or the embrace. The acceptance is wonderful. While he holds her, she's the 'other woman' (in any sense of the word); she doesn't have to put up with the looks of distrust, resentment, or open dislike.
But Jack loves Kate. So why does he kiss her?
They embrace for a long time. Juliet pulls away when she hears footsteps, not wanting to cause trouble. Kate and Charlotte emerge from the tunnel, blinking in the suddenly bright sunlight.
Jack does not protest when Juliet breaks the contact; he hurries over to Kate. Kate, almost uncomfortable with the attention, catches Juliet's eye. Juliet immediately avoids the gaze, and instead watches as he touches Kate's arm, brushes a stray curl from her face, persuades her to let him check the wound on her head.
He didn't even ask if I was ok.
The realisation is more mildly interesting than hurtful. She knows he loves Kate, and expects nothing from him. She is not remotely jealous of Kate, even though for a moment, she wishes someone cared enough about her to ask if she was ok, to check the bruise on her jaw, the split on her lip. Not Jack, though.
Because Jack loves Kate.
Juliet sighs, turning away.
This is something that she and Jack have in common.
Romeo:
A thousand times the worse, to want thy light.
Love goes toward love as schoolboys from their books,
But love from love, toward school with heavy looks.
From Shakespeare's Romeo and Juliet