As with much of Frost's poetry, "Birches" creates a mood of loneliness and isolation. Some factors that contribute to the mood include the winter weather, which seems to cut the speaker off from other people, and the speaker's discussion of the boy growing up on an isolated farm. The speaker's loneliness may be the result of adult concerns and considerations.
The speaker's loneliness is the result of the isolating advances of the modern world.
The speaker's desire for heaven is a communion with the natural world, not a desire for isolation.