Luke (DW)'s Links
Luke enjoyed hearing the Doctor talk about Aletta. She was someone that pulled him out of a dark place and truly made him happy. He wished he could meet her-- this smart, stubborn, fiery hero. When he gets his wish, it's in much more dour circumstances than he wanted, and now all Luke wants to do is disappear. Not only is the Doctor keeping the truth from her, she is already heartbroken by Luke taking her place. She doesn't deserve this. The Doctor doesn't deserve this. All Luke does is get in the way.
The Doctor fell in love with Luke instead of her. Suddenly Aletta's future as she knew it has become skewed, and she knows she must one day leave the Doctor's side. The fear is all consuming. There is nothing wrong with Luke. He is kind, patient, and has been through so much. He reaches the Doctor on a level that Aletta realizes she never did. He is her friend, and she doesn't know the anxiety she is causing him with her grief.