Why is the man making this journey?

The first response to this question is that the man is meeting a group of other "boys" at the old camp on Henderson Creek, where he hoped for "the possibility of floating logs from the islands in the Yukon down the river when the ice melted."  This was the man's first winter in the Yukon, so the reader assumes that he, like many men in this era, were looking for work, adventure, or both.  

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The first response to this question is that the man is meeting a group of other "boys" at the old camp on Henderson Creek, where he hoped for "the possibility of floating logs from the islands in the Yukon down the river when the ice melted."  This was the man's first winter in the Yukon, so the reader assumes that he, like many men in this era, were looking for work, adventure, or both.  

So the second response to this question is that the man is looking for an adventure.  Note that he goes out on the trail alone, even though more experienced men warned him against it, especially with the weather being so cold; in fact, the dog that accompanies the man even wants to resist going out in that weather.  But the man "was not able to imagine" how the cold would truly affect him.  London makes that point clear in the story when he reiterates:

The trouble with him was that he was not able to imagine. He was quick and ready in the things of life, but only in the things, and not in their meanings. Fifty degrees below zero meant 80 degrees of frost. Such facts told him that it was cold and uncomfortable, and that was all. It did not lead him to consider his weaknesses as a creature affected by temperature. Nor did he think about man’s general weakness, able to live only within narrow limits of heat and cold. From there, it did not lead him to thoughts of heaven and the meaning of a man’s life. 50 degrees below zero meant a bite of frost that hurt and that must be guarded against by the use of mittens, ear coverings, warm moccasins, and thick socks. 50 degrees below zero was to him nothing more than 50 degrees below zero. That it should be more important than that was a thought that never entered his head.

This weakness leads the reader to understand that London is not only commenting on the man's weakness, but the weakness of man in general--that explains why the character is never given a name throughout the story.  Naturally, the lesson to be learned is that Nature does not discriminate; it will take any man's life who is foolish enough to go into the cold without the proper supplies and equipment.