I'd say it's as simple as: you want her, she will pop into your dreams. Chances are your mind drifts to her as you fall asleep.
Someone wants a new dog, they will dream of a dog.
Someone wants a relationship, they will dream of relationships.
Someone wants to see this or that, they will dream of this and that.
If you want something bad enough, your dreams typically reflect that if you start having reoccurring ones about whatever it is you desire.