Physically, it's a matter of stress.
Carrying a heavy load, stresses the body. This applies both literally and figuratively.
Some forms of happiness are spiritual. These can result in epiphany or spiritual revelation. And, at times, this can result in a "lightness" which is entirely non-physical -- a floating above the physical. Sometimes this occurs with physical trauma and/or drugs, as in during surgery. But sometimes this can occur merely by a spiritual realization. This happened to me once. Seeing without physical eyes proved to be just as clear as with my human pair. And I felt the lightest I've ever felt -- in fact, floating in air.