This is not a post about Death, or the afterlife, or heaven and hell.
This is a post about a lonely god.
Before Zeus and his siblings took over Olympus, they were all swallowed whole at the moment of their birth by their own father, Cronus. Cronus had received a prophecy that his son would kill him and take over his rule, and big daddy was having none of that teenage shenanigans, so instead of family therapy he decided to just nip all possibility of being overthrown in the bud. By eating all his children as soon as they were born: girls too, because you can never be too careful.
Rhea, his wife, got tired of losing her children and managed to trick Cronus into eating a stone instead of Zeus when the last baby was born. The story of how Zeus came back and became the ruler of Olympus is another tale...what's important here is that Zeus was the BABY of the family. His oldest brother, the god* who spent the most time languishing in the darkness of daddy's belly?
Hades is the only God I can find in myth who is BOTH the oldest AND the youngest boy in his pantheon. Hades was the firstborn son of Rhea and Cronus, and last regurgitated by Cronus. Whether it's this experience or just his nature that makes him the serious God is hard to say, but he's decidedly
So, fast forward to Zeus freeing the rest of his family. Then the six of them overthrow the rest of the Titans and take over the world. To avoid future conflicts, they decided it'd be a good idea to just split up the inheritances between the boys. As his powers were over water, Poseidon took over the seas. Zeus, with his lightning rod personality, took over the skies. Earth, personified as Gaia and more powerful and older than all three of them, was equally shared.
That left the underworld for Hades.
It's important to note that Hades is NOT Death. Death is not a God: Death...ie the actual loss of life, is a force of nature. Thanatos is often the Greek personification of Death, who is the collector of life. Hades is KEEPER OF THE DEAD and Ruler of the Underworld (afterlife).
Contrary to the recent Titans movies, Hades doesn't appear too upset about ruling over the dead in his mythology. In fact, he seems to prefer it. Compare Hades' behavior to the other gods' and you'll find a more serious, quieter entity. Hades doesn't sleep with anything he can get his hands on: he doesn't have a bunch of bastard demigods running around. When petty jealousies and silly conflicts over apples and territory cause Greece and Troy to attempt to annihilate each other, he doesn't get involved other than to welcome the valiant dead warriors to their rest. In fact, in most accounts Hades prefers to be left the hell alone by his living relatives.
Personally, I think this is due to the way his life started: he spent the longest time of all his siblings imprisoned in Cronus. He's also the oldest: psychologically the oldest is often the child with the most responsibility instilled in an early age. Did Hades feel he had to take care of his siblings when THEY were swallowed as well? Who knows...but his behavior after gaining freedom is not that of a frivolous God. In fact, of the six Olympian Gods (in order of birth: Hades, Poseidon, Demeter, Hestia, Hera, and Zeus), Zeus (the baby) is the most reckless.
He takes his role as keeper of the dead very seriously, and not in the Christian idea of punishment. Hades the Realm consists of a few key areas with five rivers flowing around and between. Tartarus is what we would now think of as Hell: it's a place of punishment for those who've earned it. The Elysian Fields, or Elysium, is where souls who've earned peaceful rest go: it's the equivalent of Heaven only with more frolicking (as far as I can tell). Elysium began as a place for only demi-gods and heroes, but morphed into a place of virtuous rest later in myths.
When the dead descend to the underworld, they are ferried across the first of the five rivers by Charon, the Ferryman. In some myths the first river is Styx, in others it's Acheron. In ancient times, the coins put on the eyes of dead at burial were for their ferry toll: it was believed that without coin Charon would leave the spirit on the shores of Styx, and they couldn't get to the Underworld.
Acheron - River of Woe
Cocytus - River of Lamentation
Lethe - River of Oblivion
Phlegethon -River of Fire
Styx - River of the Underworld (often in myth Styx the river that surrounds the Underworld).
The next entity they pass is Cerberus, the three-headed Hound of Hades. Cerberus and Hades seem to get along much in the way any pet and owner do. There are tales of Hades visiting and patrolling with Cerberus, and Cerberus serves two essential guardian functions for Hades. He keeps the living out, because the living have no place in the Underworld, and he keeps the dead IN. Cerberus is feared and hated by both sides, which puts him in the position of lonely guard dog, standing alone at the gates.
Just like his master, Cerberus has a lonely existence. They're both quite solitary creatures: even after Hades and Persephone marry they're only together three months out of the year (the tale of Hades and Persephone is a post and a half all by itself, but suffice it to say Persephone spends the winters with her husband in the Underworld).
And yet, they seem to prefer their duty over silliness, and mostly solitary existence over constant noisy parties, wars, and the general cacophony of life. Maybe Hades was the original Olympian introvert. He was definitely the serious older brother to the rest of the Olympians. Personally, I've always wondered if Persephone appreciated the three months of cool quiet with a God and his Dog.
*Hestia, Demeter, and Hera were all born before Hades, to technically Hestia spent the longest time in Cronus's belly. However, this post is focused more on brother-to-brother power dynamics, so I skipped the ladies.