Serenity wants it to end. She knows she loves Endymion and that he feels the same, but she can't stand the sneaking around.
The constant looking over her shoulder as she escapes to Earth and tries to enjoy being with Endymion, and the guilt she feels whenever she looks at one of her Senshi. She can't even look Venus in the eye anymore.
But just seeing Endymion makes her so unbearably, indescribably happy. Even though there's always a knot of guilt and worry sitting in her stomach, she needs to go back to him and just bask in his presence.
More than anything, Serenity would like to reveal their relationship to her mother, but for now she knows it is impossible.
So she resorts to sneaking out of the palace and on to Earth. It hurts her that meeting with the man she loves is illegal, and she's contemplated leaving him so many times.
But the second she's wrapped up in his arms, Serenity knows she can't do that. She just loves him so much, and for just a few brief seconds, every worry is pushed out of her, and she wants nothing more than to stay like that for the rest of her life, safe and secure with the one she loves with no royal duties or obligations hanging over her head.
They aren't the moon princess and the Earth prince, enemies.
They're just Serenity and Endymion, lovers.
In that moment, it's enough.