He felt so alone. His loved ones passing away while he didn't age a day. Seeing generations of friends and their families disappear. He was all alone. No one else was like him. No comfort offered. Just discarded like a wrapper after its use. Saving the wizarding world didn't friends all turned on him. It hurt.
The Weasleys, Hermione, Neville, even Luna. All convinced he would become the next Dark Lord, or if not, brainwashed by Dumbledore. He didn't die like everyone thought. He came back after the Final battle and took credit for everything he didn't do.
The so called leader of the light just left him there, to fight his own battles, ever since he was 1 year old.
Being abused and worked like a slave since he could walk, fighting a shade of the greatest dark lord since Gellert Grindelwald and a troll at 11, fighting a basilisk at 12, nearly losing his soul to dementors at 13, forcibly used to resurrect Voldemort at 14, losing his only fatherly figure at 15, seeing his "mentor" "die" at 16 and forced to kill a man at 17.
He couldn't take this anymore. The people he thought friends betrayed him. He was shunned by wizarding society, even suicide wasn't an option.
He couldn't die.
He was the Master of Death trapped in a living hell of a world, never aging, unable to get away from it all.
When a hand reached out to him in the depths of him mind, where the horrors and dankness thrived, he took it. And then fell unconscious.
Magic looked on at the scene in worry. This young man was the only one who possessed magic that he didn't abuse. Such a kind hearted child, suffering through all of this, but never blaming it on anyone but himself. He needed to be healed, loved, treasured.
The last anyone saw of Harry Potter in his original world was a bright white light encasing him in a loving manner.
Wasting away when you couldn't die was about as close as you could get to you soul being sucked out by a dementor while still having you soul and body.
Years of solitude could be seen in the haunted green eyes of the huddled figure Carlisle came upon on the road.
With a look of understanding in the eyes that showed hardships of the world, he silently tugged the lifeless figure across his shoulders and embarked on the journey to find more in need of his help.
A few years later, the unknown figure to Carlisle, but knowing everything about Carlisles' life, being a vampire and all.
Carlise was staring at the seemingly lifeless figure he carried from the battlefield. The young adult only looked about 17 in age but the age in his eyes showed so much hardship that would be impossible for a young boy to accumulate in 17 years. There was an air of loss, hardship, pain, hurt, anger and depression. So depressed the human was trapped in his own mind, lost to the world.