It is important to note the way that Hawthorne deliberately presents the family in this excellent story as being very happy and contented together, in spite of the storm outside and the threatening mountain that looms over them. Consider the way Hawthorne presents the family to us:
The faces of the father and mother had a sober gladness; the children laughed; the eldest daughter was the image of Happiness at seventeen; and the aged grandmother, who sat knitting in the warmest place, was the image of Happiness grown old. They had found the "herb, heartsease," in the bleakest spot of all New England.
Note how all of the family are presented as being perfectly contented with their lot. However, the arrival of the guest with his wide and far-ranging ambitions changes of all of this. He causes each of the family in turn to consider how they will be remembered and what acts will commemorate their life. An important quote comes just before the landslide that kills them all:
"Old and young, we dream of graves and monuments," murmured the stranger youth.
The arrival of the guest therefore causes the family members to become unhappy with their lot in life and to begin to consider what their life amounts to. The irony is of course that by leaving their house, the place of their contentment, they actually all die. However, it is the family that is remembered and the guest, with all of his ambitions, that is forgotten. Hawthorne seems to be suggesting it is better to live a quiet and happy life than to seek to leave your mark on the world.