PHIL: I know your face so well I could have done it with my eyes closed.
After he lives the same day a few thousand times, Phil realizes that he truly loves Rita. And it's not just because she's beautiful. It's because she's the kindest person Phil has ever met. Or, in other words: she's his exact opposite.
PHIL: No matter what happens now or for the rest of my life, I'm happy now because I love you.
One thing that Phil learns from reliving the same day is to live entirely in the present moment. And one huge effect of living in the present is the realization that he loves Rita.
PHIL: On the contrary, Nancy. I love you. I've always loved you.
Early in the movie, Phil is only interested in using his knowledge of Groundhog Day for selfish purposes. In this case, he uses his knowledge to trick a woman named Nancy into sleeping with him. He even lies to her and says he loves her, which is about as unethical as you can get.
RITA: I guess I want what everybody wants: career, love, marriage, children.
When Phil asks Rita what she wants out of life, she answers that she wants what everyone wants. She wants to have a good career and she wants love and a family. For her these wants are universal, although the old Phil Connors wouldn't necessarily agree. He would define "what everyone wants" as money, power, and pleasure.
RITA: And he plays an instrument and he loves his mother.
Rita has a pretty clear vision of who her "ideal guy" is, and she surprises Phil with some of her details. For example, her ideal guy knows how to play an instrument and loves his mother. So what does Phil do? He spends the next several years getting extremely good at the piano.
PHIL: Why not? I love you.
RITA: You love me? You don't even know me.
PHIL: I know you.
The first time Phil says he loves Rita, she doesn't buy it. From her perspective, she has only gotten to know Phil for one day. Little does she know that Phil has spent the last several years' worth of Groundhog Days learning everything he can about her.
RITA: Is this what love is for you?
PHIL: This is real. This is love.
RITA: Stop saying that!
Phil quickly realizes that convincing Rita to love him will be a tall order. One of the biggest reasons is because he still thinks of love as a form of manipulation. His strategy is to use his knowledge of Rita to trick her into sleeping with him. But by the end of the movie, he redefines love for himself and instead acts kindly toward everyone around him; not just Rita.
RITA: I could never love someone like you. You only love yourself.
Rita is a lot more perceptive than Phil gives her credit for. Even though he knows nearly everything about her, she can still see through his games and she realizes that he's still out for himself deep down. Phil can change his behavior all he wants, but Rita will never love him until he changes his soul.
PHIL: You're kind to strangers and children. And when you stand in the snow you look like an angel.
Over time, Phil finds more and more reasons to love Rita. And the thing he loves most about reliving Groundhog Day is the part where he gets to tell Rita how wonderful she is and how much he loves her.
PHIL: Larry will come through that door and take you away from me, but you can't let him.
Phil knows Groundhog Day so well that he knows exactly when Larry will enter the diner and asks Rita to come with him. But Phil wants to spend the rest of his day with Rita no matter what, even though he knows he'll wake up the next day and everything will be reset. He wants to spend his time with Rita because when he's with her, all he cares about is the present.