Help! My Characters Are Rebelling!

Aaargh! I so didn't mean for...oh, wait. I might as well warn you that the next things I'm going to say are **SPOILERS!!** and you shouldn't read them if you would rather be surprised when my books get published. When, not if. =)

Okay. So, the two characters that are bugging me are Arimathea and Abriel.

Arimathea

age: 14
height:5'5"
build: On the slimmer side, although not quite wispy.
Personality qualities in brief: Forgiving, optimistic, content, unshaken, trustful.
Personality faults in brief : Forgetful, tends to overlook other's faults so much that she would trust a traitor.
Brief Bio: Raised as an orphan in Draelaet, she comes along on the quest to close the portal and proves to be an invaluable member of the group. A follower of Light.

Abriel

age: around 16
height: 5'11"
build: fairly average, okay for athleticism but not excelling athletically.
personality qualities in brief: Sensitive, protective of loved ones, strong leadership and organizational skills.
personality faults in brief: Able to detach himself emotionally, selfish.
Brief Bio: No one knows very much about him, but he claimed falsely to be a prophet and tried to lead his traveling companions into traps to accomplish tasks given to him by Dark.

And this is my problem.

You see, Abriel was supposed to marry Skye (one of my other characters) and Thea was either going to remain single or marry Erri (another character). But...

...Abriel and Thea are falling in love! I wrote a scene of dialogue between them and they matched perfectly. Just awesome. But...aargh.

I suppose this is a good sign, that my characters are real to me and that I can't force them what to do. If I could, that would mean my characters are weak. But still.

Why. Must. They. Be. Like. That?

Sigh.