Unrequited love allows you time and effort to build up something in your head. Let it marinate enough in your head and it bleeds into your heart and you become more invested even without any real push from the object of your affection. Eventually you become way more interested in winning over your love than you are in seeing if they even feel the same way, so the whole time you have basically lived a romance mentally and whether they say no or you never get to it in time, your mental romance gets crushed and you out here crying real tears.