When most people and animals pass they go to the light however some don't, for many different reasons, and linger on the Earth plain.
Some lost souls can spread melancholy and a feeling of uneasiness whilst others are quite benign. A spirit does not have to have lived in your home to be there now, they might be attracted by you or your childrens' energies. They might just have come back with you from the shops, in your car!
All lost souls should be given another chance to go to the light and once they catch a glimpse of what awaits them they go quickly, however some need a gentle prod!
Rather like those 'sticky buds' that we see when out walking. It doesn't matter how much you try and avoid them there is always one stuck to you somewhere!
Attachments or detrimental energy forms work in a similar way. Unresolved conflict creates them i.e. an argument, jealousy, unkind words etc, and once formed they can attach themselves to another persons energy field or body. I believe that these 'black blobs of energy' can cause people to act out of character and have a very negative outlook on life.
Once found they need to be cleared to allow the person to breathe and to be themselves again.