Maybe it's because I only got into this series last week, so all the earlier chapters are still fresh in my mind, but my theory is that it depends on what the recently deceased does when he or she sees the image of all their family.
So far, we've seen this with the original boy in the beginning, March, and Gugu. In volume one, after the nameless boy dies, he continues on walking into that phantom group. Where as, we see both March and Gugu turn back to the real world (and later see their ghosts following him).
On Jeannanda Island, the one time we see Fushi leave a shell behind, was of his first boy form, which didn't come back to life. I'm thinking of he were to do that with March, Gugu, Tonari, Ricard the owl, the bear, or maybe even Pyoran, it's possible things will turn out differently.