The soul is what animates the physical body. When the soul pulls out of the body, the life force goes with it.
The tradition started in the Vietnam era out of concern and respect for all of those soldiers who died and those who were MIA or POW’s.
The fact is that some souls come in and don’t intend to stay for a long life time in the physical body.
Know that you will re-connect to your loved one in the future on the higher planes.
I suggest you wait to mourn until at least 7 days after the death. Give the soul time to process it all. Just tell your self you will grieve after their soul has safely crossed over. This is their time, not yours. I do know this is easier said than done.