First, remember that the first and last scene of the novel mirror one another. The pool is the setting of the first and the last chapter of the novel. This means that the pool signifies much more than just a “place to be”. In this case, the fact that it plays center stage at the beginning and at the end shows signs of foreshadowing. Foreshadowing is the foretelling or predicting that, likewise, the pool will be the “witness” of the beginning and the end of George and Lennie’s last adventure together.
In chapter 1, George and Lennie camp by the pool prior to showing up to work at the ranch. There are many reasons why George would have wanted to camp out there rather than continuing the walk to the ranch. Judging from the tone of the scene, as well as from Steinbeck’s own style, it is arguable that stretching out the time at the very place that would be the stage of Lennie’s merciful death at the hands of George in a near future would add solemnity, or gravitas, to this particular stamp when seen in retrospect. Ironically, on its own, the pool is a symbol of safety. It is the place to go whenever Lennie and George need to meet alone, in private. This makes this moment in the timeline of the men’s lives all the more tragic.
This being said, never take Steinbeck at face value. There is much more in what is left unsaid than in what is actually laid out to the reader. One must analyze the correlation between action and context (as with every piece of literature) but also add a degree of doubt in the motivations of these characters, as they represent broken, isolated men who are impoverished in more ways than one.
Let’s analyze then.
…”we’re gonna sleep here because I got a reason”.
He later argues with Lennie because the latter does not want to spend the night and would rather go straight to the ranch which is straight ahead.
"No reason at all for you. I like it here. Tomorra we're gonna go to work. I seen thrashin' machines on the way down. That means we'll be buckin' grain bags, bustin' a gut. Tonight I'm gonna lay right here and look up. I like it."
Fine. We could go the simple route and say that George simply loves nature and wants to “look up”, as he says. But, why? He has been alone with Lennie plenty of times and surely he has looked up a lot. There is no reason to spend the night by the pool, especially if there is supper to be had at the ranch. Moreover, George and Lennie will get in a lot of trouble the next day for being late.
Let's then look into plausible theories.
a) Could it be self-sabotage in George’s part? Likely. Broken men and women tend to make things worse for themselves as their way to indulge in negative self-fulfilling prophecies. When they think their lives are bad, they do things to test fate and see if they could get worse. It is morbid curiosity.
b) Could it be a want to postpone the inevitable? Also possible. George is tired of Lennie. He knows that Lennie will forever be his “ball and chain”, so to speak. He knows that he holds no future (again, a part of the broken man paradigm). Therefore, he is likely to believe that things will go badly, yet again, in the new place. Really, when have things even gone right for George and Lennie? Why assume that a miracle will happen now, when they are older, more tired, and more desperate than ever?
Still, we could summon a completely different theory, as to why the pool must be an important setting, if we look into the author’s purpose. Steinbeck made the exposition at the pool. He established all that needed to be known about the characters, their past, their traits, and their potential future in the very spot where everything will later on end in chapter 6. Therefore, an isolated place had to be created at the beginning and at the end.
In order for things “to go as planned” (for the author, not for the intentions of the characters), there had to be an alpha and an omega. Therefore, creating the same setting for the beginning and the end would have required that the entire first part of the novel begins and ends there, just like the ending will.
Regardless of whether you go with the author’s purpose rationale, or the character traits rationale, both are easily arguable and there is enough evidence to support both possibilities.