To be honest, I don't really see Cassandra and Cullen getting together either, but it's definitely not because I'm too attached to them with my characters. I just think it's because they have different romantic needs. Cassandra can be pretty rough and harsh, but she secretly wants to be treated like a lady, and she expects her man to step up, ignore her tsundere tendencies, and give grand romantic gestures (flowers, candles, poetry, romantic picnics, the works) anyway. Cullen may be romantic too, but he's much more shy, timid, and insecure in love.
I didn't really notice until my second playthrough romancing him, but Cullen's always worrying about "getting it right," and frequently rubs his hand against the back of his head, stutters and stumbles through his words, and constantly needs to be re-assured that he's doing fine and she still loves and values him even though he thinks he's doing a poor job. Based on what I know about Cassandra, she doesn't seem to have any patience for this kind of namby-pamby behavior. She doesn't seem to be the type who'd want to hold his hand through the wooing process, or constantly coddle or reassure him he's doing fine. She seems to want someone who'll take the initiative and show confidence with wooing her. And Cullen is not that bold or confident in love. (I mean, the female Inquisitor basically has to initiate all flirting and spell out how she likes him before he asks her out.)
Don't get me wrong, I love Cassandra. I still lament that she's not romanceable for women, and one of the best romances I experienced in the game was my male Lavellan romancing her. Again, it just comes down to this: I think Cassandra and Cullen want different things. Cassandra wants someone bold in love, while Cullen is shy and insecure in love. Cullen needs someone supportive and nurturing in love, and Cassandra... gives tough love.