1. You see a ladder, what do you do? You climb it to see what's at the top. He obviously climbed it to some extent to get some of those rungs. Why wouldn't he have gone all the way?
2. Say he did go all the way up. He sees what's on the other side. We don't know what it is. It could be anything. Maybe it's a zombie world. Maybe there's a serial killer. Maybe there's demons. (I know, those are a bit far-fetched. But still. They work.) Whatever it is, maybe that wall is the only thing protecting him from whatever's on the other side. And he wants to stay alive and warm as long as he can. So, naturally, he builds a fire.
Because sometimes in life there's ostacles that get in the way(addictions and such) that stop us and hold us back from our potential. Not every one is this way but still
2. Say he did go all the way up. He sees what's on the other side. We don't know what it is. It could be anything. Maybe it's a zombie world. Maybe there's a serial killer. Maybe there's demons. (I know, those are a bit far-fetched. But still. They work.) Whatever it is, maybe that wall is the only thing protecting him from whatever's on the other side. And he wants to stay alive and warm as long as he can. So, naturally, he builds a fire.