I think that it sometimes seems that George and Lennie have a parent/child relationship with George as the often rather despairing parent. George clearly feels responsible for Lennie and there is some suggestion that they may be related somehow. George is all too aware that Lennie would be unable to cope on his own and does his best to keep him out of trouble; although clearly this is not always possible.
Lennie is often compared to an animal; a bear in some places that does not realise its own power. George's role is to protect Lennie from this power and to shield others and when he fails to do this he resorts to killing him as he knows he could not cope with the consequences of killing Curly's wife.