Death's Door

Approximately 12,000 years, 7 months, 3 week, 2 days and 46 minutes, that was how long Death had been guarding the gates to the afterlife. An infinite amount of souls, waiting to pass into their deaths awaited Death's attention. With a heavy sigh, Death beckoned the next soul forward, a little girl.
With a quick glance, Death flicked over her file.
Name: Cerberus Plutonia Smith
Age upon death: 12 and 1/2
Cause of death: Neck snapped from jumping off a cliff into the surf while on a school trip.
Time of death: Premature passing. 4:02pm, 02/07/18

The file went on, listing all the girl would've accomplished if she hadn't died, who she left behind, her goals, aspirations, highs and lows, summary of life and so on and so forth. Death sighed heavily. He was so tired of reading about "Premature" deaths. Nevertheless, Death had no choice but to beckon Cerberus forward. Before he had a chance to pass judgement, the girl spoke.
"Can I ask you a question?" Came the questioning voice of the girl, looking up towards Death's frightening face.
Death sighed, already knowing the question that Cerberus, like many others before her, was about to ask. "What comes next?" Or "Why did I have to die so soon." Really, there were a good many questions Death was tired of answering; nevertheless, the rules of passing specifically stated that any spirit between life and death may ask death at least one question before passing on.
"What is your question?" Death asked with a resigned sigh.
"Do you ever feel pain?" She asked somberly.
Death was taken aback. In his thousands of years of guarding the gates of death, he had never been asked this question. It was as if someone had released a wave of sadness within him as he pondered over the female's question. No one, dead, living, or immortal, had ever asked Death about himself. Humans and immortals alike avoided Death like the plague- probably partly because Death invented the plague- so it shocked Death that a passing human might take interest in what he felt.
Finally, after what seemed like an infinity of moments, Death responded.
"...no." He admitted, but his answer sounded more like a question than an assured answer.
"No?" Cerberus repeated, surprised by the answer.
"No. I am a special bred of immortal men. The gods designed me not only immortal but undefeatable. My skin is harder than iron, my strength unparalleled, and my systems so superhuman that no poison or plague can ever effect me. I cannot die and thus I cannot feel the pain of death or any pain of any other kind."
"What about empathetic pain?" She asked again, oddly curious.
"Empathetic pain?" Death asked, the concept tastings foreign and murky on his tongue.
"Yes, empathetic pain. Say someone dies prematurely or their death is excruciating, do you ever feel pain for them?"
Death grew quieter. The rise and fall of his eerie chest stilled, his eyes fluttered weakly, as though a single question could be the impossible death of him. Finally, in a sullen whisper he responded to Cerberus.
"I used to." That was all he said for a good many minutes. The line of passing spirits grew longer and longer as they waited to meet with Death, yet, as he spoke with the little girl who made him think, the millions of other passing spirits seemed to drift from his mind.
Death opened his mouth once more. "I used to," he repeated hoarsely, "During my first thousand years, I would cry bitterly as the children paraded by me into their afterlives. I would try to comfort them as their round, ashy little faces contorted in fear, realizing they were dead. I would hold my breath and bite my tongue as the leaders and hero I know must live to win human kind's wars died before it was their time. Then I grew numb." He paused, staring unseeingly at the long line of wavering souls. "Occasionally, from age to age some deaths would strike me as horrendous and I would feel pain for the dead and dying. During what you humans call 'The Holocaust' I felt immensely for the spirits of humans were trapped and dying, yet they didn't fully die. The victims during that time would cling to life, barely wavering between their 'camps' and my domain. Their torturous deaths would be prolonged for months. I wanted to take them, to relive them of their earthly torture and welcome them into the peaceful nothingness of my dominion, but humans seem to believe that nothing, even torturous camps, is worse than death." He stopped then. Halting in the middle of his story to stare at the girl with his hollow eyes.
Cerberus gazed up at Death. She thought that he has beautifully sad eyes... if you could call them eyes. They were pools of the darkest black, lit only by dim skies. To Cerberus, Death's eyes resembled the darkest and clearest night's sky. He was mournfully lovely.
"How do you bear it?" Cerberus asked. She felt only a small piece of Death's pain, and yet it overwhelmed her.
"Because I must." He responded, somberly. "No other being has the powers of death. Who else could lead souls to rest?"
"So you will spend the rest of infinity helping wavering souls find rest, yet never obtaining peace yourself?" She spoke the question quietly, sadly.
"I must."
"Why must you do it alone though?" She questioned. Death paused, thinking. For once he didn't have a real answer.
"I suppose because none of the immortals are willing to blend with death." He responded unsurely.
"Well, then, why doesn't a mortal keep you company?"
"Human souls will eventually be destroyed if they stay in my presence for too long. It is not possible."
"You're the mighty Death. Make it possible." Cerberus challenged. "I had no interest in relaxing during my life and I have no interest in eternal peace. If human souls are the problem then turn my soul into another form."
Death was stunned. In his thousands upon thousands of years he had never thought of turning a passing human into a different form of soul to keep himself company.
"Do you realize that you are asking to be transformed into another being that will suffer in the same incessant occupation as I myself do?"
"If you're saying do I realize that I'm asking to be Death's friend and 'suffer' with him eternally, yes. I have no better plans for eternity." Cerberus said with an honest shrug.
"Well then... what type of being would you like your soul to be transformed into?"
"Something menacing yet...friendly, like you."
Death nearly laughed. What a contradictory child.
"How about a viper? That's menacing."
"No thank you, I never liked snakes."
"How about a lion?"
"No thank you. Lions are too... golden."
"A spider?"
"Too creepy." She said with a shiver.
"A wolf?"
"Too wild.... but, maybe a dog?"
"A dog? Are you sure?"
"Yes," she responded confidently, a silly smile crawling up her childish face. "A three headed dog."
"So be it." Said Death.
And thus Cerberus the 3 headed dog that guards Death's door was born. Wether or not dog is really man's best friend, Cerberus is, was, and always will be Death's best friend.