Pharaoh as the connection of people and gods
At first only pharaohs and their families were considered to be worth immortal life. It was considered that the king has the god's origins. That is why it was natural that people believed that after death pharaoh will return to the house of gods to his father. But there was also additional sense in it.
The pharaoh that was the god was the connection of sky and earth and his power was the base of the social stability and the peaceful order. He had to be immortal because he guaranteed unchangeable sunshine, fertile lands and everything that brought prosperity to the country. Later people started to unite pharaoh with the activity of the most important gods.
With the development of such religion, pharaoh was getting more and more god's traits. When pharaohs were united with Osiris, the burial ritual was democratized. Osiris became the symbol of dieing and resurrection that was proper not only for the pharaoh but also for all people. And pharaoh turned from the immortal symbol into the symbol of immortality.
If to remember the story of Osiris, then you'll find out that Osiris was the son of the sun god and had difficult life. During his life he was good and kind and was bringing knowledge to people being the ruler of the Ancient Egypt. The same qualities had pharaoh. Naturally people started to unite these two images until the final one was created that made pharaoh the part of the human world and the part of the god's world at the same time.