When you lift weight you are fighting against a gravitational force. Gravity pulls the weight down but you want to push or pull it up. Rep after rep, set after set.
In the end, fatigue sets in and the earth wins. The earth always wins. So you are fighting not to beat gravity but to keep yourself going. Why? So you can grow stronger. Without resistance you can’t improve. Without obstacles you can’t get better. Gravity is your friend.