This particular type of illusion is what psychologists call a projection. That means that you have parts of yourself that you are not aware of. You do not see these parts in yourself, but you see the qualities of these parts in other people. The person you feel you have fallen in love with is a person upon whom you've projected parts of yourself you're not willing to acknowledge. As you become acquainted with yourself, you have less and less objections.
Illusions are like filters.
As you remove illusions, you remove those filters. There are many types of illusions, but this will give you an idea of one type. Many of your experiences in the Earth School are illusions in this sense. Fear, anger, resentment, jealousy, depression and other painful emotions do not exist outside of the Earth School. Yet these painful emotions are extremely valuable because they lead you to parts of yourself that you need to heal.