Sometimes You Get What You Need
She tells House there is no chance. They will never happen. They will never be an "us".
She hates every word of what she tells him and while she believes in her mind that it is the right thing to do, her heart does not.
She doesn't love Lucas. She has feelings for him that have grown over the time they've been dating and that she thinks could turn to love with enough years, but that is all there is to it. He is a nice man but she doesn't love him.
Even so, she is convinced Lucas will make an excellent partner. He is trustworthy and reliable and there for her and Rachel. He isn't afraid to commit to her and he's very sweet. She likes him and he worships her. He is everything she needs.
House is not.
To throw away her relationship with Lucas for him is not the sensible thing to do, no matter how much a part of her wants to. House is unreliable and can't be trusted. They have an understanding and sometimes he proves he does have a heart and pure intentions but those times are too few. He can be sweet, but more often than not, he is manipulative and dry and sarcastic.
He has a side to him no mother wants their child to grow up around. The side that shows he cares is too hidden, too rarely seen.
House isn't worth the risk.
It hurts her to do it, but she says to him what she can to dash his hopes. They can never be.
She hopes he doesn't realise that she's trying to convince herself as much as she is him.
What she doesn't say to him is that her heart desires everything her head does not.