So, seeing a few Responses i let you know why i love One Piece.
Lets start with a Flashback... Back then when i was younger, i was used to watch one piece (together with Digimon and Dragonball) with my 5 years younger brother on TV. We were Kids, Boys and we fighted alot, you can imagine. We lost our Father early (he was not even 3 years old back then), so we only had our mother and our doggo. When i kinda had to watch him often, we often fighted and disagreed alot, but with these 3 Shows, we Always came together. Even when we had a fight before, got hurt, disliked each other alot, when One piece came in TV, then there was this Quality brother time, no matter what. Looking back at the time, it was what we needed.
The years came into the land, we grew up, i went 18 and left the house to study (around 400km away from home). First, we went apart from each other, cause he started to get interest in Girls, i tried to get an own life with studying and Friends and stuff (seriously, i kinda had not enough time for my Little brother back them, or didnt took enough time, still Feeling guilty). When he had then some Trouble (no Details, Police involved, but Nothing really serious) and he came to me, asking for help, because he didnt wanted to bother our mom. That was the Moment we restarted watching One Piece together. Since then, every Sunday, even Living hundreds of Kilometers away from each other, we meet in Teamspeak, or nowadays Discord, and watch One Piece together. We talk About it, we laugh, we discuss. Every Sunday. And even when i am 30 years old now, it is something i will Always do together with my brother, like we did as Kids.
So apart from this, One Piece has just great aspects of story telling. It is huge, has great world building, has so many reoccuring stuff, so much people... It is like and endless expanding world, which makes it so believable. its not just (if i take Naruto f.e.) one village and feeling like the whole world is working around naruto, or dragonball, where the only thing matters is Goku. One Piece is a living world, with an history, with stories about noland, stories about Oden, with sceneries where the maincast isnt and wont be involved, like reverie. I really love the world building aspect, but also i love how you get connected to different persons, to their history and even get emotionally involved into a ship.
So i guess there are the main reasons for me.