It really doesn't matter anymore that she broke up with Finn. She's hurt (let's face it, when the guy you love breaks up with you, it hurts, no way around it) but the more she thinks about it the more she realizes that he never loved her, not really. She realizes that he was in love with someone who will put up with all his crap (excuse her language). He loves that she can easily forgive him when he done her wrong. He loves that he can be stupid and she doesn't point it out. He loves that she'll just follow him around because she doesn't want to lose him even if he's an ass (the most fitting word) to her.
She realizes that kissing him is not a mistake but an opportunity to see her situation in a new light. She's trapped in a destructive relationship and it's killing her (literally and metaphorically). She's losing herself, her passion to strive to be the best. She's taking a backseat because she feels that it will castrate Finn of his manhood (she knows that being in the spotlight is his drug). She's not the Rachel Berry that will take Broadway part with her raw talent and passion. She became Finn's girlfriend.
But most of all, she realizes that kissing him brought back memories of something that was awaken in her. Something that she never knew existed before, the untainted emotion that she only read in romance novel. She feels alive. She feels free. She feels, she just feels.
So, it doesn't really matter anymore but what does is that she needs to have him again.
It starts out with her watching him in glee. There's something about his eyes that she finds so captivating. They are always so intense, so angry. No one knows this but even though she had her eyes on Finn when 'Babygate' imploded, she always took a second look at him. She saw the way he looks at Quinn and Finn, like he wants to rip them apart with his own hands. Such intense anger. Now, she still sees it and it's the anger in his eyes that draws her to him.
Her plan to have him again is easier to accomplish than she thought. All it takes is for her taste those amazing lips again and the rest is history. Like he always say, they are hot together and splitting them up is a sin. And she doesn't want to sin. But what he does with his hands, his lips, his tongue and his cock is another form of sin that she'll be willing to go to hell for. Finn doesn't make her feel this way, he's not even close. It doesn't feel dirty, what they are (most of the time); it's like silk on your skin, soft and sensual. It's never just rough and get-down-to-business but gentle and first-drop-of-snowflakes. It's his sensual side that keeps her with him.
He has the reputation of being the ladies man and she doesn't blame him for that (have you seen the man? It's like God's gift to women). He has the reputation of being with numerous of girls at once but he's upfront with that, it's never a relationship only hook ups and if a girl believes otherwise, it's her fault not his. But now that he's with her, no words have to be spoken to know that they are in a relationship. She just has to look at his eyes. Those once wandering green hazel eyes are now only looking at her. And it's the way he looks at her and only her that makes her fall for him.