Wilfred worried about Donna. He worried about her constantly. He worried that one day she would remember, remember the Doctor and all their wondrous adventures and that would be the end of her.
Donna never worried. She was carefree, spending time with friends, moving from one temp job to another. She never thought about, never remembered the best friend she never had.
The Doctor worried, about everything. Was another alien invasion going to threaten his humans? Was the time-space continuum in trouble? He always remembered, always thought about his new human family, especially the one that was his best friend of all time, his sister, family that he chose.
So Wilfred watched Donna. He watched her run around, laughing and bossing people around. And sometimes, sometimes he watched her watch the stars, watch the stars and dream.
And the Doctor, the Doctor ran, ran and never stopped.