Let us remember that the first dwelling that Robinson Crusoe builds is done quickly and soon after he finds himself stranded on his island. His immediate needs are to build a shelter for himself that he could use to protect himself from any savages that might be on the island or any wild beasts that could come to harm and destroy him. Thus it is that this shelter is chosen and built primarily for its defensive capabilities. Later on, of course, Crusoe finds that the earthquakes that the island is subject to makes the location for this shelter--beneath a mountain--not very wise, however, it is important to remember that the cave-like shelter he creates with its defences are for the purposes of protection above all.
However, as time goes by, it is clear that Robinson Crusoe has seen no other humans and no wild beasts that might endanger his life, and having explored the island, he finds that the interior of the island is much more suitable to live in as it contains an abudance of fruit and crops that he could use to live off. Thus it is that he explores the island and finds the ideal place for his new abode that is away from the shore:
When I came home from this journey, I contemplated with great pleasure the fruitfulness of that valley, and the pleasantness of the situation; the security from storms on that side of the water, and the wood: and concluded that I had pitched upon a place to fix my abode which was by far the worst part of the country. Upon the whole, I began to consider of removing my habitation, and looking out for a place equally safe as where now I was situate, if possible, in that pleasant, fruitful part of the island.
Thus we can see that where Crusoe makes his second dwelling is far more sensible and logical given the new situation that he finds himself in. Being on an island with no dangerous wildlife and no other humans, as Crusoe assumes, means that he does not have to worry about protection as he did at first, and can pick a location that is closer to the wildlife and crops and fruit that will sustain him. Thus, when we think about the difference between the two dwellings, we need to remember the different situations that Robinson Crusoe finds himself in as he slowly gets used to living on the island and facing the challenges that this situation brings.