I was bored so I shall try to explain the reason why this chapter in the manga made us so sad if you disagree please don't hesitate to point out what you do not agree with. So here's my explanation:
The reason why we feel sad is that we can connect to the main character, you may be going 'I don't see a where I connect to the character' and that's what makes it so good; we unconsciously connect with it. You see, the character being lonely shows us that we're lonely as well that's 1 of the connections we make; another is the journey, as he ventures across the ice land he tries to accomplish his goal. The author makes the journey and portrays it in a way in which we don't question the journey instead we connect with it unconsciously, We're all scared of being alone, never being able to reach our goal, and dying as a lonely person. Humans are made to be connected; to be as 1 within a community of people, that's what makes this chapter... no sorry, this series so good, it explains how lonely the character at the start is, we don't even get to know his name. He's just another dead boy who failed to accomplish his goal.