Lennie is mentally handicapped. He has the mind of a child, but the strength of a bull. Lennie is mentally slow and socially awkward. He wants nothing more than to pet pretty, soft things. Without George, Lennie is socially isolated because he doesn't quite know how to act in social situations. He would rather talk about puppies and rabbits. In addition, when Lennie gets scared or anxious, he has a tendency to lose control, mentally and physically. This is dangerous because he is so strong. He loses the ability to control himself with Curley and Curley's wife. Because this is always a dangerous potential, George tries to do all of Lennie's talking for him and George tries to watch Lennie at all times. So, even though George does shield Lennie from certain social situations, George is Lennie's conduit to social engagement and conversation.
But even with George's help, Lennie has a lot of trouble fitting in. He has a child's mind and a giant's body. Lennie is innocent and has no ill intentions at all. But the potential for him to become afraid combined with his brute strength is a lethal combination. This combination forced he and George to leave Weed. And a similar event occurred with Curley's wife, which also forced Lennie to leave the ranch. Here are two examples of how Lennie's loss of control leads to literal social isolation. He functions well enough in a social circle, but he seems to inevitably do something that leads to self-isolation. In other words, he fits in for a while, but his unpredictable behavior usually results in him running away. In running away, he isolates himself from society.