Unrequited love is, arguably, the inspiration behind much of the most enduring and effective works of
art ever created, be them literary, visual, cinematic, or musical. It's also something that's so ingrained in the
collective unconcious that one almost feels an innate obligation to feel it at one time or another in his or her life. Honestly, though the portion of the world's population composed by my friends and acquaintances is admittedly rather small, not one of them has gone without those pangs of wanting that someone that will not be had. This phenomenon is a result of various factors.
(1) The odd attraction people have towards feelings of pain and loss.This is, in part, borne of people's innate love of tragedy in all forms, especially the form most conducive to this particular node, transgressive love (something started with Tristan and Isolde, perfected in Romeo and Juliet, and imitated thereafter countless times). People want exciting lives, and people with exciting lives usually have intense emotions, such as unrequited love.
(2) People are attracted to things they know they can't have. Always have been, always will be. It's a lot safer to dream about something that you don't have to worry about actually coming to pass. Daydreams often usurp reality in terms of excitement, and people inevitably fall in to liking the image that they create around the object of their desires more than the comparatively bland realities that they actually live. It's not really that you love that person, it's more that you love the person you've created to go behind that face to which you were drawn. (Marcel Proust describes this in his colossal novel, In Search of Lost Time, to a far lengthier and competent degree than I can.) Basically, you don't REALLY love the person. Rather, you've fallen in love with an image of your own creation that never truly existed.
(3) (This is really just an off-shoot of 2, but having three points just makes me feel better) People get bored doing the same thing every day: work, school, what have you. Everyone needs an escape and a fantasy, and more often than not that involves some wish for companionship, spiritual union, and fulfillment. It's funny that way; love is the object and the source of so many dreams, but in order for it to actually work it must have it's foundations firmly rooted in reality.