In Chapter 3, as George tells Slim that Lennie will do anything he is asked, he relates that he used to play jokes on Lennie "cause he was too dumb to take care of 'imself." He adds that Lennie also did not realize that a joke was being played upon him. Then George relates the incident that broke him of his practical jokes on Lennie:
....Couldn't swim a stroke. He damn near drowned before we could get him. An' he was so damn nice to me for pullin' him out. Clean forgot I told him to jump in. Well, I ain't done nothing like that no more.
This passage is very significant to the moral lesson of John Steinbeck, the author Of Mice and Men. As a Socialist who wrote this novella about the need for the fraternity of men, Steinbeck points with George's tale to the necessity of those who have more to give and share with those in need. All men must help one another.
In my opinion, what George learned is that it is wrong to bully weaker people. Put differently, he learned that when you have more power than someone else, it is not morally right to use that power for your own amusement. Instead, you have to consider what is best for those who are weaker than you.
George says that he used to enjoy pushing Lennie around. He got a kick out of using Lennie to make himself feel smart and powerful. But then he ordered Lennie to jump in the water, Lennie did and almost drowned.
At that point, George learned that people with power over others must not abuse this power (this is something that Curley needs to learn but has not yet learned). George is not yet past being mean to Lennie at times, but he has certainly learned to rein it in more than he used to be able to.