George takes care of Lennie because he has promised Lennie's Aunt Clara that he will look after him and protect him. This promise and George's moral astuteness bonds the men together. Lennie would not be able to survive outside of an institution if George were not there to ensure his safety. George views his responsibility as a burden at times, but in certain moments throughout the book one may see his true affection for Lennie, particularly through their mutual vision of a farm of their own. This sadly unattainable dream allows them to live outside of the stark reality of their lives. George views Lennie as a brother, friend, and partner in crime. Without Lennie, George would be lonely. It seems as though they need each other equally, although upon first look it seems like Lennie mostly needs George. This makes the ending of the story all the more tragic.
George promises Lennie's Aunt Clara that he will take care of Lennie, and it starts out as an obligation. However, somewhere along the line, George's relationship with Lennie escalates to a level something similar to that of a parent protecting a child. George sees Lennie as a child-like figure in need of protection and help, and he leads his life in a determination to give Lennie that care and protection against the cruelties of the world.
George promises Lenny's Aunt Clara when she dies that he will look after Lenny and he very loyally does just that. Some of George's reasoning is behind his promise and obligation, but as the men grow closer to one another, it becomes a deep friendship, like brothers, that only the two understand. George is very protective of Lenny and hopes that someday they can have a ranch of their own where Lenny can tend rabbits and stay out of trouble.
George promised Lennie's dying aunt that he will take care of him.