One Piece is an astounding series. It has well developed characters with personalities all their own, fantasmic action and adventure portrayed in a believable format, and a sense of off the wall humor which leaves the masses laughing and gasping for air.
However, along with these beautifully defined characteristics, it does something which other mangas, and most comics in general, fail to do. It captures real human emotion.

The adventures the characters partake in and the personal struggles they go through are ones which any person can relate to.
While we may not stretch like rubber, fight with swords, or sprout extra appendages- we all have lost a loved one, been hurt by someone we care about, or have done something with good intentions which turned out bad.
We've all laughed, we've all cried, we've all felt emotions.
We follow the emotions with the characters best during the flashbacks, where we encounter their past experiences.

How Luffy became rubber and got his hat, how Zoro made a promise and became a 3 swordsman, how Nami came to hate pirates and love money, how Usopp became a lying orphan, how Sanji almost starved and became a chef, how Chopper lost his 'father' and became a doctor, how Robin lost everything and became a fugitive, how Franky became a shipwright and lost his mentor, how Brook 'lost' a friend and died. Every person can relate to one of these stories, some even relate to more than one.
Not on a basis of what happened to the characters, for example hardly anyone is a shipwright or a swordsman, and no one who is reading this is dead (unless you're a ghost or something)
It's on a basis of what the characters felt. The pain of loss. The hardships of life.
People who've lost a parent cried with Nami, Usopp, and Robin. People who lost a friend sobbed with Zoro and Brook. People who've starved ached with Sanji. People who've lost or hurt a loved one agonized with Luffy, Chopper, and Franky.
And yet... they keep on smiling.
There is a quote from the series, it's made by Nami's 'mother', Bellemere.
"Always remember the strength to continue laughing."

That is another thing the series does which no other can do. It gives hope.
Some of the quotes I've read/heard from this series have given me goosebumps from the power of the words they've contained.

"Always remember the strength to continue laughing." - Bellemere

"No one is born into this world to be alone!" - Jaguar D. Saul

"When do you think people die? When they are shot through the heart by the bullet of a pistol? No. When they are ravaged by an incurable disease? No. When they drink a soup made from a poisonous mushroom!? No! It's when...they are forgotten."


These quotes... are ones to live by.
Whether he knew it or not when he created this series. He created something amazing, that no other could do. He created real emotions. And he created hope.