Sacrifice Theory
Tobias once told Cassie that he would willingly die, if it was for Rachel.
Rachel once told Cassie, she was afraid of what she might do, for Tobias.
Cassie can't do anything for both of them, fragile and uncontained as they are. And Jake's eyes grow dark and clouded when she tells him these things.
Jake always told Cassie he loved her. He loved her, but-
Cassie is fearful of all the things she would never do, would never give up, for Jake. Still, she knows, he'll never dream to ask them of her.
-And that's why Cassie holds on to him, and loves him a little more.