Pluto and Jupiter and Neptune were three very powerful gods. They were also brothers.
When their father, the mighty Saturn, died, and the brothers first divided up the world, Jupiter took the sky, Neptune took the sea, and Pluto took the underworld. In time, Jupiter became the king of all the gods and all the earth and everything on it and above it and, well, just everything. Neptune married happily, and remained Lord of the Sea. As for Pluto, he was content, living in the Underworld as its Lord and ruler.
Pluto was NOT the lord of death. His job was to run the Underworld. He was a good ruler. Parts of the Underworld, where heroes' souls dwelled for a while after they died, were very nice. Parts were not so nice. Those were for people who were not so nice during their lifetime.
You might think Pluto would be terribly lonely, surrounded by the souls of the dead. But he wasn't lonely. Not at all. Unless they were in trouble with the gods, souls didn't stay around forever. They were reborn and sent back to earth. That gave Pluto a lot of different souls to enjoy or harass.
And, Pluto had visitors. His brother Jupiter visited, as did his nephews Apollo, Mercury, and Mars. Pluto came up to the surface now and then, if he had to attend a meeting of the gods or something. The thing was, Pluto preferred the Underworld. It was home.