|I should have known better than to get emotionally involved with these two.|
There is no denying the writers of Merlin did an excellent job developing their characters. Even if you plan a happy ending, you can do as they did and add an element of classic Greek tragedy to improve your story.
For example, in Merlin, Merlin's greatest virtue is his loyalty and unswerving devotion to Arthur. Unfortunately, as season five unfolds, it becomes clear that Merlin's determination to remain faithful to his destiny as Arthur's protector, at any cost, ends up being Arthur's undoing.
What is your character's greatest virtue? How could this virtue end up destroying them, or one they love?