Because this world view says that there is nothing outside of space, time and energy, naturalism insists that death brings the extinction of personality and the disorganization of matter. Things just stop living and start decomposing. You're dead, and that's it. To the naturalist, there is no life after death. The body recycles back to the earth and the mental and emotional energies that comprised the person disintegrate forever.
Pantheism teaches reincarnation, the belief that all of life is an endless cycle of birth and death.
After death, each person is reborn as someone, or something, else. Your reincarnated persona in the next life depends on how you live during this one. This is the concept of karma, which is the law of cause and effect in life. If you make evil or foolish choices, you will have to work off that bad karma by being reborn as something like a rat or a cow. If you're really bad, you might come back as a termite.