All our emotions are 'illusions', really. They are nothing but neuronal connexions in the brain and combinations of neurotransmitters therewith. There is no problem with that, as long as we are aware of it. It becomes but an illusion when we lose sight of the chaotic yet existent rodomontades within our heads.
or you may say, when you're in love, with that particular someone, or whenever you look into a mirror and the first thing that comes to mind is that significant other, and suddenly the idea of her makes you like or love yourself that much more for loving her. That my friend is when you know you're truly in love.
"I don't want to live with a woman, I want a woman I cannot live without!"
"Live life as if there were no tomorrow, but love as if you could love forever!"