Why does love sometimes hurt?

In this world, true pleasure is derived from meeting challenges and succeeding, and that way we can feel "independent" and Godly (as He is the ultimate in independence). In other words, there needs to be the possibility of occasional pain and defeat, in order to feel and appreciate the thrill of victory and love. Judaism says that pain isn't the opposite of pleasure, but rather enables (!) pleasure, e.g. children are our greatest source of pleasure but also, our greatest pain and disappointment. In the round world which God created, the further you get from one point, the more you return to that same point, so what seems opposite or contradictory, really is not.