Do people have intrinsic value? If so, then there must be an afterlife. The value a person has is due to them being made in the image of God. If made in the image of God then they have incredible lasting value which lasts into the afterlife.
The first reason is an argument from authority. The vast majority, of peoples, religions and cultures have all believed in life after death. It is innate in us to believe it. We become historical snobs by rejecting the notion without completely examining why so many people believed it.
The second reason is there is a desire to conquer death so, it must exist. We cannot desire that which does not exist in one form or another. We desire money because it exists, We desire heaven because it exists.
The indestructible nature of a soul. People die even of natural. causes. Yet, they are not suffering any illness or disease. They just lose their souls. The soul also cannot be added or to or taken away from. It has a supernatural quality of immortality.
We have rational thinking and the ability to choose between right and wrong. We can act above our physical bodies in a spiritual manner. Spiritual acts demand spiritual actors so, we are therefore above our bodies and have life beyond the physical body.