In the book the face on the milk carton, why does janie keep having daydreams?

Janie has daydreams for many reasons. She sees a little girl on a milk carton and thinks it's her and daydreams about who are her real parents and the people she thinks are are parents must be kidnapped her. She also has daydreams about her kid's names (denim and lance), she daydreams of having a drivers license too.  If you need something specific let me know.  I hope this helps.

That is basically it what the other person said :)