In Crimson Peak, why did Thomas Sharpe fall in love with Edith Cushing?

It's the same as asking "How can somebody fall in love?"

Still if we try to answer it from the film's perspective, one thing to be noted is that it will be purely speculative as we don't know the other contenders (his ex-wives). Still the bulletin points of speculation I can get are:

• He never got intimate with his previous wives.
• She was prettier than the other ones. Or she has more innocence to attract him.
• He is tired of all these murders and his sister.

Otherwise we can just say that his sister never gave him space to fall for any other woman but this time he got it and fell in love with her.

Thomas chose Edith. Even when Lucille warned Thomas against choosing her. Lucille saw Edith as a legitimate threat to whatever it was they had. She was young, beautiful...brilliant. Even worse, Edith saw Thomas for more than what Lucille had envisioned for him. A prisoner in their mansion locked away together. Eventually, Thomas saw that too. But he was too much of a fool to stop it.