People often say things they don't mean. George is no exception. He does get frustrated with Lennie just as any parent would with a disobedient child. Sometimes, Lennie acts like a disobedient child. He does not necessarily try to be annoying. He just lacks the mentality level of an adult.
George often criticizes Lennie, but he is doing so out of concern. He realizes the dangers in which Lennie can find himself.
Remember, as the novel begins, George and Lennie are on the run for Lennie's inappropriate behavior in the previous place they worked. Lennie would not let go of a young lady's dress. He frightened the young lady. Lennie has the mentality of a child. While he actually means no harm to anyone, he is strong. When he holds on to anything or anyone, he can be too physically over powering.
George loves Lennie and he has dreams and plans to buy a home with Lennie. He wants Lennie to have his rabbits. He repeats the dream for Lennie's benefit. When George yells at Lennie, he is only trying to protect him:
...he yells at Lennie from time to time...But George is essentially a good man.
One way to know how much George needs Lennie is evident when he has to shoot Lennie. It is extremely painful for George to have to shoot Lennie, but George realizes that Lennie's punishment for killing Curley's wife will be more than Lennie or George can bear. George loves Lennie and when Lennie is gone, George calls off the dream of buying a home. He tells Candy the dream is over. This proves that George enjoyed sharing his dream with Lennie, and without Lennie, the dream will not be the same:
George is loyal in his friendship with Lennie, and he is also remarkably pure of heart. When George is driven to shoot Lennie after Lennie accidentally kills Curley's wife, he destroys his own dream, too.
George will be lost without Lennie. He never meant it when he said he would be better off without Lennie.