Sometimes hearts become heavy, this doesn't necessarily mean that they are unhappy. It simply implies that they have become weighed down by love. They've lived life and, although their bearers have tasted misery, still survived to breathe another day. That's special. If you've never cried tears of true sorrow then you haven't smiled with true happiness either.
Hearts become heavy because love isn't like helium. Love doesn't float in the clouds, it is weighty like metal. That's what makes love so precious, because like gold we cannot afford to lose it. You see, because of love's heaviness, we notice when it's gone.
Sometimes hearts become heavy, this doesn't necessarily mean that they are unhappy. Light hearts are lonely hearts, they've nothing to grasp onto. There is nothing to attach their insubstantial forms to reality, and so that is why some hearts are heavy. It's because they are alive.