Love.
Unless two people are in love with each other, you can't really FALL in love because that person that you've "fallen" in love with isn't there to catch you. This is also known as one-sided love. It sucks, especially, when the one you're in love with doesn't (and may never) be in love with you. And like I said, you don't FALL in love. More like you drown in it. You're hoping -- or eve praying -- that one day someone will be there to save you from those waters of not just love but pain and loneliness that seems almost impossible to bare. Sometimes, that person does come to save you. But it seems pretty rare.
You, yourself, are probably trying so hard to get past this thing called love but can never escape from it. Sometimes people delude themselves into thinking, "Hey, one day he/she will come around." But that's pretty unlikely.
A lot of the time, love hurts. Sometimes it's just at the thought of BEING in love. Sometimes, the one you love hurts you whether it's physical, emotional, or mental. Love just hurts. I mean, I could be wrong, of course. And this IS just out of personal experience.
But then again . . . this has happened more than once.