Disclaimer: I do not own the legal copyrights to Supernatural, nor am I someone who has any affiliation with anyone involved in producing the show. This is done for fan purposes only, and I have no intention of making a profit off this story.
Demons never enter purely romantic-motivated relationships with humans. It's just how life works. If a demon and a human are involved, the demon wants something. The demon uses the human. That's how things are, demons aren't secretly waiting for love to change them. Ruby isn't waiting for love to change her. She doesn't want to be fixed. Sometimes she remembers how life was when she was human, but that's so long ago that she can't fathom how she was ever the fourteenth-century pathetic creature she'd been back then. She much prefers the twenty-first century. She rather enjoys the innovation of indoor plumbing.
Sometimes, Ruby lies to Sam when they are together. It's not like she has something against him, more like she has to help start the Apocalypse. Nothing personal, really, but she's got a job to do and that involves getting Sam Winchester to love her. If she gets involved in a romantic relationship with him, he will become easily manipulated, and she can use him for the Apocalypse. He has powers. They will be of use.
She doesn't consider herself a monster; she's just a demon, and the Winchester brothers kill her kind. To her, that makes them more monstrous than her own kind. So demons lie, and cheat, and have a tendency to make deals that backfire on the signer. It's just the way they are, and she wishes Sam would understand that. She's good at pretending that it doesn't bother her. She's been a demon far longer than she's been human; pretense is second nature to her by now.
"Do you ever feel remorse about what you've done?" Sam asks her. Ruby doesn't quite understand why he wants to know this; she's wondering if he wants to pretend she's a human. She won't ever be human, Sam should know this, but he seems to forget right now.
She cocks her head, pouting in the way she does when she wants Sam to forget that he's asked her something she doesn't want to answer. "Just drink, baby, and your powers will develop..." she says. She cuts herself with a knife, letting the tempting demon blood well out of her flesh. Sam latches onto the blood, letting out soft mewling sounds like he's already too far-gone in his addiction to let what he'd call reason sink back in.
When they're done with sex and blood for the night, Ruby whispers, "Sometimes. Not often. It's just what I am. Do you ever feel remorse for what you've killed, regret for the person you once were?"
Sam just nods, glazed look in his eyes like he's too high to even think about most anything. Ruby has managed to wedge herself into his life; she wonders when Sam will figure out that she's using him. His brother is in Hell. He's desperate for someone to help him understand his powers. So she tells him what he wants to hear. She has to. If she says anything else, she thinks that he'll leave. Lucifer won't like that, and Ruby will become demoted or otherwise harmed if Sam Winchester doesn't fulfill Operation Apocalypse.
It's probably the least-imaginatively-named plan ever, but this is Ruby's job, and she has to do it. She thinks Sam would understand if he ever learned why she used him. If his brother ever comes back from Hell, though...that's something she isn't prepared for. She thinks that she'll have to double down on her lies to Sam, pretend that she's still got her humanity...pretend that Dean doesn't understand his brother. More than that, she has to act like Dean is no better than her. She thinks she can do that; spending any amount of time in Hell has negative consequences on a human.
She doesn't feel any regret when she leaves out the window, promising herself that she won't return until there's need of her. Of course, it won't happen that way, but Ruby knows how their relationship is. If Sam wants to pretend—to himself—that Ruby is his girlfriend, she's been more than fostering that impression.
All she needs to do is bide her time. She needs to wait until circumstances are right. Ruby can wait.