he's seen hundreds of workers with "a little piece of land in his head" who never attain their dream. However, Crooks's opinion changes once Candy assures him that they almost have enough money to purchase the property. Crooks then offers to join in their dream by saying that he could contribute and lend a hand.

As the only Black worker on the ranch, Crooks is discriminated against and forced to live by himself in a small room instead of the bunkhouse with his peers. Crooks resents his lowly, oppressed position on the farm and is extremely lonely. He laments about his situation to Lennie by telling him,

A guy goes nuts if he ain't got nobody. Don't make no difference who the guy is, long's he's with you. I tell ya...I tell ya a guy gets too lonely an' he gets sick (73).

Given Crooks's difficult situation on the ranch, the prospect of living off the land with his friends and having a place to call his own is a refreshing idea. Crooks views their dream as an escape from his unfortunate situation as well as an opportunity to experience a fulfilling life. Crooks can see himself living among his peers, having a voice, and enjoying the freedom of being his own boss. Candy and Lennie's dream offers Crooks a second chance at life and the opportunity to exercise his independence.

Candy is motivated to contribute to George and Lennie's venture of owning a farm because he is growing older, has lost his faithful dog and friend and is now isolated, and may soon lose his job sweeping out the bunkhouses.

The old, stoop-shouldered man, Candy, does little but push a broom and clean the bunkhouse since he is too old and crippled; added to this condition, Candy has lost a hand in a farm accident. Therefore, he is worried that he will suffer a similar fate to his old dog, or at least, be alone.
Further, when Candy overhears George and Lennie talking about the farm that they hope to have, he eagerly asks them, "You know where's a place like that?" George is at first suspicious, but Candy expresses his worry that he is not "much good with on'y one hand," and he adds that he received money for his accident and can contribute over three hundred dollars. Certainly, at the prospect of a real home and fraternity with men rather than isolation, Candy eagerly asks to participate. 

Throughout the novel, both George and Lennie are propelled by their desire to own their own ranch one day. They dream of a day when they can be their own bosses and leave their itinerant ranch hand life behind. They claim they are different from other workers because they have each other and something to look forward to. George says,

"Guys like us, that work on ranches, are the loneliest guys in the world. They got no fambly. They don't belong no place. …With us it ain't like that. We got a future. We got somebody to talk to that gives a damn about us.” Lennie broke in. "But not us! An' why? Because... because I got you to look after me, and you got me to look after you, and that's why."

Yet George is conflicted by his desire to live a life independent from Lennie, and he threatens Lennie with leaving over and over. George also feels obligated to help Lennie because of promises he made to his family. George genuinely cares for Lennie, but when Lennie becomes a liability, he must make a difficult choice.

Lennie does not have much choice in the novel but he knows that he is dependent on George for survival, so he tries to please George and keep their dream alive. His dream of taking care of the rabbits shows that Lennie just wants to be loved and accepted. Ultimately, he seeks this same love and acceptance from Curly’s wife, but it leads to a series of tragic events.

Your question does not make it quite clear what, specifically, the external and internal motivations are for, so I will discuss the factors which drive their relationship and their desire for independence.

As far as their relationship goes, it is obvious that Lennie and George share a close bond. On an internal level, they enjoy having company and being able to communicate. It is good to know that someone else 'has your back.' The two men are attached to each other because they share a common interest and take mutual delight in discussing their plans. Furthermore, they actually like each other. Each cares equally about the other. Even though George is at times quite harsh and Lennie threatens to leave him, it is obvious that neither really wishes to break company with the other.

External factors which affect the relationship and lead to its creation are the fact that George promised Lennie's aunt Clara that he would take care of him. He is, therefore, bound to his pledge. George sometimes expresses resentment about having to bear this burden, as in the following excerpt from chapter one: 

God a'mighty, if I was alone I could live so easy. I could go get a job an' work, an' no trouble. No mess at all, and when the end of the month come I could take my fifty bucks and go into town and get whatever I want. Why, I could stay in a cat house all night. I could eat any place I want, hotel or any place, and order any damn thing I could think of. An' I could do all that every damn month. Get a gallon of whisky, or set in a pool room and play cards or shoot pool..." 

"An' whatta I got," George went on furiously. "I got you! You can't keep a job and you lose me ever' job I get. Jus' keep me shovin' all over the country all the time. An' that ain't the worst. You get in trouble. You do bad things and I got to get you out." His voice rose nearly to a shout. "You crazy son-of-a-bitch. You keep me in hot water all the time."

George is clearly frustrated, but does not truly mean what he says, though, and later apologizes to Lennie when the latter threatens to leave.

"I been mean, ain't I?"

"If you don' want me I can go off in the hills an' find a cave. I can go away any time."

"No- look! I was jus' foolin', Lennie. 'Cause I want you to stay with me..."

A second external factor that motivates the two to maintain their relationship is the fact that their closeness makes them different than all the other ranch hands who don't have the privilege of close friendships. The other men are lonely and drift from place to place, never forming any close ties. To George and Lennie, their association makes them special, as is evident from the following passage:

George went on. "With us it ain't like that. We got a future. We got somebody to talk to that gives a damn about us. We don't have to sit in no bar room blowin' in our jack jus' because we got no place else to go. If them other guys gets in jail they can rot for all anybody gives a damn. But not us."

Lennie broke in. "But not us! An' why? Because... because I got you to look after me, and you got me to look after you, and that's why."

The two men share a dream to live independently. They both have a deep desire to obtain land and be masters of their own destiny. Internally, this is probably a universal motivation; every person wishes to be in charge of his or her own life, free of another's control or command. The dream is so powerful that it empowers the two men and deepens their attachment.

Lennie expresses his desire in simple terms: he wants to live "off the fatta the lan'" and tend rabbits, while for George it means that he can decide to do whatever and whenever he wishes. He can work when he wants to and entertain guests and run his life just the way it pleases him. The ideal is a powerful one, and both have a clear visual image of their shared future.

External factors which drive the dream are the drudgery of their existence; they want to be free and in charge, instead of having to rely on others for their existence. Having their own place would mean they can take care of themselves. Secondly, they would be landowners, something that other men in their situation would never have. They would be respected and would be able to make choices about who they did and did not want for company, a choice they do not currently have as ranch hands.

It is tragic, though, that their ideal never comes to fruition, for fate intervened and, in the end, their hope was destroyed.