When Lennie says that Curley's wife is pretty, George tells him "You leave her be". Here George is warning Lennie to get away from that woman because he knows from Lennie's eye that Lennie is attracted by her beauty, as being attracted by other beautiful and soft things before. This shows that Lennie doesn't have a sense of judgement for his action, he have no idea what his action means, and what serious consequence such as losing jobs or getting caught they may cause, but George does. Every moment, George lives in clear consciousness of what life he is living, but he can't fight back, all that he can do is to continue because he needs to raise money for their future farm. Sometimes ignorance is a bless when your life can't turn to be better, and from this perspective George's wit makes him sympathetic in Of Mice and Men.
The second reason why I sympathize with George is that because of Lennie's mental handicap he has to be Lennie's "father" and take care of him for everything. "You never oughta drink water when it ain’t running, Lennie". This shows that George even tells Lennie what can eat and what can't. It's a typical father-and-son model. So we can see from this sentence that George cares about Lennie in every little thing.
Also George is Lennie's guardian. When Weed's incident happens, George tells Lennie to hide in an irrigation ditch until people who search them give up, and then they left the town. And then George finds a new job for both of them, he keeps the work card for Lennie because Lennie can't "keep it safely". When Curley is playing difficult with Lennie, George protects Lennie like "Lennie didn't do nothing to him". All these suggest that to George, Lennie is like his son, and it is the father who has to take care of the son and help him out of trouble. Everything about Lennie, from what to eat to what to do, hangs on George.
At that time, taking care of oneself is hard for a worker, let alone two, and one is mentally handicapped who has no idea what needs to do and what should not to do. Let's assume that George is alone and will he be easier on his life? Probably yes. If there is only George himself, he can handle the work easily with his wits, and just like he says "Live so esay, without you on my tail".
But he also says "You get used to to gain around with a guy and you can't get rid of him". So to George, though he also complains about Lennie, he has never thought about leaving Lennie alone. He says to Slim he likes Lennie's company. He would look after Lennie the same way if Lennie didn't die. He volunteers to be Lennie's father though the naughty son causes him really trouble.
The real burden Lennie brings to George and his willingness to carry this burden make me sympathize with him more and any other character.
The last reason I sympathize with George is that he has to make the cruel but the only decision: shoot Lennie himself. This is a tough decision, but it is the only right thing to do at that time otherwise Lennie would be killed brutely by Curley or be held in a dark room with a dog chain on his neck. So it is better to kill Lennie peacefully, and by George himself.
But how painful it is to make and finally do that decision? "The hand shook violently, but his face set and his hand steadied", this indicates that George is going through a fierce struggle deep inside. Lennie is George's friend, the only friend in the world, he is George's relative, closer than real blood. How could he just kill him?
And when George finally puts the trigger, he "sat stiffly on the bank and looked at his right hand that had thrown the gun away". This shows that he can't stand the loss of Lennie, and until that moment does he really realize that Lennie is gone for good. In this lonely world, all he has is Lennie and he killed him with his own hand. What's worse, he has to go on his life, with clear consciousness of what he has done, and in his rest life, he will be tortured by Lennie's haunting face. What a poor man!
In conclusion, I sympathize with the character of George because he knows clearly what he is going through but has no power to fight back, and the real burden Lenned gives him plus his willingness to take on this burden, and of course his shooting to Lennie. His tragic ending makes Of Mice and Men better than other novels with similar themes.