Actually, that's quite an interesting way of putting it.
You are saying that someone has such emotions tied up with the memory of a dead loved one, that their mind creates a realistic projection of that person, which seems real to them.
OK, let's go with that theory for a moment. My question would then be: are most 'ghost' sightings of loved-ones, or are they of strangers? For example a traditional haunted house where the ghost is allegedly that of someone who died hundreds of years ago.