I read in movie trivia that Mace had developed feeling for Cassie but never acts on them and as a result he dreamed of her every night. (Night is such a funny concept in space, isn't it?)
He only ever dreams of her.
It's beautiful and innocent, the picture of her in a dress, walking slowly on the seashore, waves washing her feet, her face basking in the sunlight, warm and bright, and non-existent.
He never allows himself to think about her.
He doesn't think of her watching crashing waves in the Earth Room.
The image of her lifeless body never crosses his mind, when he finds out there won't be enough oxygen for them to survive.
He doesn't picture her while being stranded on Icarus I.
He doesn't run for her, when he finds out about the intruder.
So when the Sun is dying, when all what's left for him is to freeze and turn into stardust, he allows himself to think of her.