To add a bit more here - As CSC and Ethan have pointed out, there's A LOT written about this most interesting and important scene in other threads because it sets the tone for so much of the entire film. My own abridged version of an answer is:
1. They're teenagers. Even in 1963 teenagers (especially males) weren't known for an abundance of social graces, especially when raised in remote, sparsely inhabited locales.
2. They don't really know who they are or why they're there. They may actually be competitors for the same (single) job. They have no way of knowing that they'll end up working together as a pair.
3. Ennis in particular isn't very talkative and his insecurity (partly due to his confused sexuality) keeps him from initiating any contact even though he was there first. Jack, as we learn, is much more outgoing and probably would have made a gesture if he had any kind of acknowledgement from Ennis upon his arrival.
4. Finally, it may be difficult for others to understand the "cowboy" culture of the American west where self-reliance and personal independence were (and are) cherished. Not to be confused with being rude, impolite, or hostile, it's a different view of where a man stands in the world and in relation to others. There is a point of pride in being able to survive on your own in nature's elements, and, to some extent, the need, or even the desire to be social is considered a sign of weakness. We see this as the boys try and prove themselves to one another on the mountain. Not until they each have conquered several adversities do we see them really open up and relax to one another.
Of course please keep in mind this is JMHO, there are many other viewpoints and interpretations to this wonderful scene.
