Sometimes Shiv thinks that she genuinely loves Tom. When she sees his excited reaction to being handed a prenup (it means we're actually getting married) or the way he loves Mondale. Other times, when she sees him kissing up to her father or letting her brothers humiliate him, she despises him. The two emotions sort of meet in the middle until she thinks, yeah, this can work.
Ultimately she doesn't want to turn into her mother, married to someone with more power than her. She needs someone just beneath her. She doesn't love him. But she can stomach him. Just.