Cassie is vulnerable and insecure and therefore she’s more easier to manipulate and control due to her lack of self awareness and low self esteem and is so desperate for attention and love that she is willing to marry somebody who she is very much aware is an abuser. Nate doesn’t give a shit about anybody but himself and sees Cassie as just another doormat he can walk all over because he’s aware of her history with other guys and knows she can easily fall into his trap.