We aren't actually given a reall explanation for why Tom chooses to embark on a relationship with Myrtle. However, we can definitely infer a number of reasons as to why he wishes to pursue it by examining his character. Tom, throughout the novel, is presented as a man who likes power and wants to have power over others. He is described as arrogant and a bit of a bully. He is also used to having mistresses, presumably as a way of manifesting that power or arrogance or sense of superiority over others. What is really interesting is that he picks a woman who is from a working class background and is married to a weak an ineffectual husband, the exact opposite of Tom. It seems that part of the attraction is that he can satisfy his need to feel superior by giving Myrtle the taste of luxury and "owning" her whilst cuckolding her husband without his knowledge. Likewise, I think we can safely infer that he doesn't love Myrtle. To him, she is just another object for him to smash and use, as Nick says about him, and he does definitely "smash" her when he breaks her nose. Thus we can argue that the relationship is more about questions of power and superiority than love.