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The answer you are looking for can be found in Chapter 8. After spending three days by himself on the island, Huck is shocked to discover that somebody else is living there as well. When he discovers that it was Jim, the slave of Miss Watson, he is delighted to see him, once he has managed to convince Jim that he is not actually a ghost who has come to haunt him. Note how Huck responds to discovering he is sharing the island with Jim:
I was ever so glad to see Jim. I warn't lonesome now. I told him I warn't afraid of him telling the people where I was.
Thus we can see that Huck is very pleased to find Jim, because he now no longer feels lonely and isolated. To be able to share his solitude with somebody else makes it that much more bearable.