"It's intelligent creatures you got to watch out for." (7.80)
There are a lot of creatures—human and animal—that just bump through life without giving it thought. Gus, as an intelligent creature himself, knows that it's the ones with intelligence who have the most potential.
Life was finally starting, Newt thought. (11.77)
Newt wants a life of adventure. Some men want a life of relaxation. Those men can stay in Lonesome Dove. Newt feels most alive when he's on the trail.
[Deets] had known several men who blew their heads off, and he had pondered it much. It seemed to him it was probably because they could not take enough happiness just from the sky and the moon to carry them over the low feelings that came to all men. (19.111)
This is a dark little tangent about suicide, something that doesn't come up a lot in Lonesome Dove. But it shows us how some people's happiness comes from within. Deets is like that. Jake Spoon isn't. In a way, Jake commits suicide by acting in a way that gets him killed. He never finds a satisfying way to live.
"Well, here's where we all find out if we was meant to be cowboys," Augustus said. (31.1)
"It ain't dying I'm talking about, it's living," Augustus said. "I doubt it matters where you die, but it matters where you life." (46.36)
This is ironic, considering Gus does care where he's buried. But maybe Gus's motivation is to make Call have an adventure of his own and return to Lonesome Dove. When Gus dies, Call can live. But does he?
Thus the sight of the road of bones stretching over the prairie was a shock. Maybe roads of bones were all that was left. (54.91)
Gus ponders his own insignificance. According to him, all he—and everyone, really—is is a bit of bones that will end up buried in the ground. This is a recurring image.
"I don't sing about myself," Campo said. "I sing about life. I am happy, but life is sad. The songs don't belong to me." (60.23)
Po Campo has a balanced view of life. He's comfortable being happy or sad, and just taking whatever life gives him. When life gives him grasshoppers, he fries them and eats them.
"Yesterday's gone on down the river and you can't get it back." (58.45)
More life-as-a-river analogies from a cowboy. Gus is definitely the go-with-the-flow type, as we see toward the end of the novel, when he accepts his own death without much of a struggle.
"Well, life's a twisting stream." (56.64)
You've probably heard this kind of thing before, but it doesn't sound so clichéd when it comes from a cowboy who has crossed more than his fair share of twisting streams.
It struck [Lorena] that endings were never as you would expect them to be. (92.22)
Ain't this the truth? Life takes a bunch of unexpected turns. Lorena never expected to be living with a family in Nebraska—and being happy about it. She also doesn't expect Gus to die. Like Call, Lorena realizes she can't try too hard to control things. She has to make the best life out of the hand she's dealt.
All his life [Call] had been careful to control experience as best he could, and then something had happened that was forever beyond his control just because he had wanted to find out about the business with women. (46.80)
Call learns that he can't really control his experiences. He should listen to Gus's river analogies, which we have below, to understand that sometimes you just have to roll with life. The more Call tries to control things, the worse things seem to go for him.