why?

Basically Marlow realized how difficult it would be to achieve their goal of finding Kurtz.

"Next thing he wanted to know 'how long it would take to' . . . I interrupted him again. Being hungry, you know, and kept on my feet too. I was getting savage. 'How can I tell?' I said. 'I haven't even seen the wreck yet--some months, no doubt.' All this talk seemed to me so futile. 'Some months,' he said. 'Well, let us say three months before we can make a start. Yes. That ought to do the affair.' I flung out of his hut (he lived all alone in a clay hut with a sort of verandah) muttering to myself my opinion of him. He was a chattering idiot. Afterwards I took it back when it was borne in upon me startlingly with what extreme nicety he had estimated the time requisite for the 'affair.'