What does Steinbeck say about friendship in Of Mice and Men? What types of friendship are present? What are some quotes that prove this? What would be a good thesis for this?

As the previous educators have discussed, the most notable example of friendship in the novella is the one between George and Lennie and Steinbeck makes it clear that we all need friendship in our lives. However, Steinbeck presents friendship in a more negative light, too. The friendship between George and Lennie, for instance, ends in tragedy when George shoots Lennie to save him from the mob. In addition, Curley’s wife is so desirous of friendship in her life that it damages her reputation on the ranch and, more importantly, leads her to death in the barn with Lennie.
Perhaps, then, we could argue that Steinbeck views friendship as a necessary evil. On the one hand, friendship stops people from feeling empty and lonely, but, on the other hand, friendship can place people in dangerous situations. This idea could form the basis of a thesis statement about friendship in Of Mice and Men.

Steinbeck presents friendship as an essential part of being human, especially for those on the bottom rung of society who don't have much else in life.
George and Lennie's friendship is, of course, the most notable in the story. As itinerant workers, they're unable to establish any lasting friendships apart from the one that binds them together. This makes them ever more reliant on each other for mutual support. As they'll never stay in one place for any appreciable length of time, there's no opportunity to make more than casual acquaintances. But that's not a problem as George and Lennie have each other, as well as their dreams of one day running their own ranch.
Slim notices the close friendship between this unlikely pair. As he keenly observes

Ain't many guys travel around together.

This brief quotation highlights just how unusual the friendship between George and Lennie really is. Slim's an old hand, and has pretty much seen it all, but he's never seen anything like the close bond that exists between this unlikely pair.
A possible thesis statement could be "Friendship in Of Mice and Men allows the poor and exploited to hang on to their dignity and self-respect in the midst of widespread poverty, hardship, and exploitation".

Steinbeck says that friendship is very important in Of Mice and Men. He shows that what gives George hope of a better future is the possibility of owning a small farm with Lennie. Even though George sometimes gets irritated with Lennie, he depends on him for companionship, both in his life as a migrant worker and in his aspirations for a better future.
In contrast to George and Lennie, the other migrant workers are isolated and lonely. Yet many of them are attracted to the idea of a farm and the companionship and rootedness it would offer. Even Crooks is drawn to the idea, though he quickly backs away from it as an impossible dream.
A quote that illustrates the friendship Lennie and George share comes near the end of the book when George is about to shoot Lennie to save him from a worse fate. George says to him "you ... an' me," meaning they are friends who will always stick together. George then says, speaking of the farm and the shared future they will never realize:

"Ever’body gonna be nice to you. Ain’t gonna be no more trouble. Nobody gonna hurt nobody nor steal from ‘em.”

This shows the dream of a world of kindness and fellowship.
Finally, George affirms the deep friendship he feels for Lennie:

“No,” said George. “No, Lennie. I ain’t mad. I never been mad, an’ I ain’t now. That’s a thing I want ya to know.”

A possible thesis would be: "Steinbeck shows that friendship and solidarity with others is key to building a better life than that the migrant workers experience in a soulless capitalist system."