I remember that once Nearly Headless Nick told Harry that souls find a chance to come back to the world or go the other way (or something like this). Those who choose to come back or have an unfinished business come back. For example, Moaning Mertyl came back as a ghost to haunt those who bullied her.

Many people in the Wizarding World has some unfinished business. They can come to fulfill those. Death Eaters like Bellatrix could come to get revenge. So why isn't the Wizarding World filled with ghosts?

  • @LethalCarrot I don't think this is a duplicate. Harrys parents were sensible to embrace death while others nay not be – the-profile-that-was-promised Nov 30 '18 at 12:21
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    Maybe Bellatrix is around somewhere... ;) – Mal Nov 30 '18 at 14:52

Becoming a ghost is a difficult decision, not everyone is ready to make it.

From Pottermore, emphasis mine:

Muggles cannot come back as ghosts, and the wisest witches and wizards choose not to. It is those with ‘unfinished business’, whether in the form of fear, guilt, regrets or overt attachment to the material world who refuse to move on to the next dimension.

Having chosen a feeble simulacrum of mortal life, ghosts are limited in what they can experience. No physical pleasure remains to them, and their knowledge and outlook remains at the level it had attained during life, so that old resentments (for instance, at having an incompletely severed neck) continue to rankle after several centuries. For this reason, ghosts tend to be poor company, on the whole. They are especially disappointing on the one subject that fascinates most people: ghosts cannot return a very sensible answer on what it is like to die, because they have chosen an impoverished version of life instead.

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