Kallian loves the feeling of freedom. She breathes the open airs of the Fereldan wilderness with much delight. Two out of three treaties are done; the last one, with the mages, should be a breeze.
She has freedom to go where she wants to, love whom she loves, do what she wants. There are limits, true—she must defeat the Blight, for one thing, by fulfilling the Warden treaties—but at least, she can do these on her own terms.
She can also go drinking while she's at it. She swears she was intoxicated while transversing the Brecilian Ruins. Is it her fault that the grateful Dalish gave her a packet of herbs meant to be smoked with special pipes, or that the pompous dwarven king gave her a stash of his best brew—the brew she occasionally shares with the dwarf warrior? And anyhow, didn't she break the werewolf curse, get rid of a pompous Dalish lord, and get said pompous dwarven king a silly throne, all in exchange for their armies against the Blight?
The love thing is her favorite perk. She can choose, for the first time in her life. No elders telling her whom to marry, no shem lords demanding her womanhood whether she likes it or not. Not that Kallian accepts just anybody's love. She took Alistair's rose, but found that she could not be with someone so… human. That rules out Leliana as well, especially when she mentioned that in Orlais, elves are little more than pretty slaves. She still has nightmares of the horrible shem pig Vaughn, despite the fact that she has gutted him and claimed her vengeance.
Zevran, on the other hand, knows what freedom, and love, are. Knows how precious they are, in that tone that belied great sadness about that other elf assassin. If Zev could love once, he could probably do it again. Or not. There's no certainty in that. Anyhow, not even Kallian could dictate on his heart.
And that is what makes his affection more precious. It is free, and it is uncertain.
Its uncertainty makes it all the more beautiful.