According to the NOAA, the answer is that technically, lightning is a two-way phenomenon. The part that is visible to the naked eye goes from the ground back up to the sky, but only after a path of negatively charged electricity makes its way down from the clouds.
Negatively charged particles come down from the clouds in a series of bursts, and positively charged particles rise up to the top of tall objects, attracted by the above negative energy. When the two charges meet, a channel is developed through which lightning travels. The process occurs in about one-millionth of a second, too fast for the human eye to see the strike forming.