When you can’t find what/who you would love to have, you start making him/her/it up in your dreamland, you start fantasizing about having him/her/it around your and enjoying him/her/it. After a while the realistic side of you wakes up and threatens you that you’re going crazy, that you should stop considering your dreams so real and live like a normal sane human being! And you, who are freaking out about turning into an insane thing, deprive yourself of the dreamland, try to forget all about him/her/it and keep yourself busy with serious sane real stuff.
But suddenly in your normal sane life you see or hear something that reminds you of your made-up person/thing, reminds you of your dreamland and the joy you used to have there, and you miss him/her/it!! You literally miss someone/something that doesn’t exist! You miss the unreal!