How is Slim lonely?

John Steinbeck presents Slim as a singular character who is sufficient unto himself. He is a skilled rider and driver and holds seniority on the ranch. Compared to the nomad-like workers, Slim is a fixed point. He seems to be satisfied with his situation.
Steinbeck places Slim into an almost supernatural category. Slim seldom judges others but has an innate sense of fairness, even justice. He need not make demands because he is naturally respected. Slim had a

gravity in his manner and a quiet so profound that all talk stopped when he spoke . . . . His slow speech had overtones not of thought, but of understanding beyond thought . . . .

Slim has been compared to a priest. He listens to others more than he speaks and picks up on subtext. George confides in him.
Steinbeck does not explicitly name loneliness as one of Slim's qualities. Clearly, he is man apart. We get a sense that he is settled within himself, accepting of his position.
If we read Slim as lonely, we accept that self-sufficiency is undesirable or inadequate. Slim, as Steinbeck paints him, is peerless. He has no matching other half or even close male friend, yet he does not seem aloof. If Slim spoke of desiring company, we might see loneliness. It rather seems that he is just alone.

Slim is lonely in the sense that he is completely different from those around him. He is the voice of reason and restraint amid all the turmoil and conflict on the ranch. People automatically look up to him in a way they simply don't do with anyone else. His evident wisdom and dignity make him widely respected, and it is difficult to find anyone on the ranch who can command similar levels of respect.
But Slim is also lonely in more conventional terms. He doesn't appear to have anyone you could call a friend. The men approach him with their problems, trusting him as a confidant, but there is nothing really deeper between them. However, when Slim comforts George he shoots Lennie, telling George that he's done the right thing, we sense that this loneliness may change. There is a suggestion here of a possible future friendship between the two men, one forged in mutual respect, understanding, and shared experience.