She's a teenage girl with raging hormones. Teenage girls fall in love with every guy they like but it's simply puppy love and infatuation. Not real love in the sense that an adult woman would make such a solid decision about loving someone.

she wanted some old man dick

She's a 17 year old high school girl. One week she thinks that she's in love with one guy. The next week she's all over some other guy.

I never liked the way their relationship ended, but I suppose it was rather predictable, albeit bittersweet. At least the guy got to live out an old man's fantasy, so he's got some good memories to take to the grave.

She did love him and he loved her. She simply realized what Michael Caine was trying to tell her all along and that was that there was no way in hell it could ever work out. He was married and she was his best friends daughter.
I the end she walked up to him and said something about him being right about it being a dream, a wonderful dream. She still loves him but she understands there is no way they can be together.