There's a One Piece promo song, sung by the Straw Hat Pirates' voice actors, called "Family" or "Oretachi wa family!" ("We are family"). It plays during the ending of some TV specials and was even popular enough to get a dubbed version from the Funimation cast. What's special about this song is how it perfectly captures the Straw Hat's relationship in its lyrics: "We're not acquaintances. We're not friends...We are family! (While we're out at sea!)." The Straw Hats aren't just friends out at sea, they're a found family, and One Piece does a great job of reinforcing this classic storytelling trope.
A found family, or family of choice, is a group of companions who form bonds strong enough that they consider each other a true family. One Piece mangaka Eiichiro Oda has written countless different characters and storylines around the theme of "found family." One strong example is Nami, who was raised by the ex-marine Bellemere alongside her adoptive sister, Nojiko, after Bellemere found them in the aftermath of a battle. Of course, there's Luffy and his adoptive brothers, Ace and Sabo, who cared about each other so much, they toasted with sake as kids to prove their loyalty as brothers. Oda loves exploring this topic, of people finding their true family. Interestingly, it always comes back to the Straw Hat Pirates, who best exemplify this heartwarming theme.
The Straw Hats all refer to each other by their first names with no honorifics. A running gag with Luffy is that when he meets someone new, he makes up bad nicknames for them, such as Traffy for Trafalgar Law or Mingo for Doflamingo. It's interpreted that Luffy is bad at remembering names and this helps him, but Luffy never refers to his crewmates by nicknames, only their given names. You would think an easy-going dreamer like Luffy would be the type to refer to crew with dumb nicknames, but it's the opposite, as his family is who he wants to commit to memory. The other Straw Hats also refer to each other by name because they see each other as equals, especially the two female members, Nami and Robin.
Representation of women in shonen manga is a mixed bag. Sometimes the female characters just exist to be the love interest for the cool hero character (Orihime from Bleach, Sakura from Naruto), but there are also better developed characters (Lisa Lisa from JJBA: Battle Tendency, Makima from Chainsaw Man) and occasionally even female leads (Emma in The Promised Neverland, Jolyne in JJBA: Stone Ocean). Suffice to say, One Piece 's female characters are well-developed. This is partially because Oda actively doesn't write romance in One Piece, which means Robin and Nami are free to be their own characters. Both are equal members of the crew, and theirabilities are never called into question.
Oda puts a lot of thought into all of his characters, but this is especially true of the Straw Hats. Oda approaches character motivation by giving each crew member in the straw hats a dream or goal alongside going out to sea with Luffy: Zoro wants to be the World's Greatest Swordsman, Chopper wants to be able to cure any disease or illness, Sanji wants to find the All Blue, etc. This is great for two reasons: one, these dreams give personal motivation to why each member decides to travel and join up with Luffy, and two, it shows they aren't simply defined by the Straw Hat Jolly Roger, with goals just as important to them as Luffy becoming the King of the Pirates. All of them have a stake in making to the end of the Grand Line, but the great thing about Oda is he knows to put the time into the little distractions and relationships along the way as well.
Whether it be the rivalry between Zoro and Sanji, or the love of academia Chopper and Robin share, Oda crafts strong human relationships between the crew. Fans have given the title of Monster Trio to Luffy, Zoro and Sanji whenever they handle a threat as a team, and the Adorable Coward Trio of Nami, Usopp and Chopper as they usually hang back and hide. The classic color spreads of One Piece also do an excellent job of portraying the Straw Hats as a group of individuals who find comfort in being around each other. Pick out any of Oda's color spreads and you'll find a sense of joy and camaraderie among the Straw Hat Pirates as they partake in an activity or exploration of a new environment that way a closely-knit family would.
It's when things are at their most dire that we see the lengths that the crew will go for each other. Enies Lobby is still remembered as one of the series' best arcs. The Straw Hat crew invaded the government judicial island to save their Nakama, the last archaeologist of Ohara, Nico Robin, a woman told for the good of the world the best thing she could do was die. Once cornered by the government assassins CP9, Robin turned herself over in exchange for her friends' safety. But finding this out didn't give the Straw Hats any sort of comfort, rather it incited the determination in them to go and save Robin, leading to one of One Piece's best scenes, as Robin, CP9 and the Straw Hats all face each other on the roof. Despite everything they've had to face, they're still here to save their friend, and when Spandam taunts Luffy by saying that trying to save Nico Robin is making an enemy of the World Government itself, this doesn't deter them either. In an act of defiance, Luffy and Sogeking set fire to the World Government flag, willingly declaring war on the World Government if it means saving their friend.
All of the Straw Hats have felt ostracized or lost at some point, but going along with Luffy into the Grand Line gave them a chance to not let that source of pain define them, but inspire them to go after what they always wanted. Even in its darkest moments, One Piece has always been an uplifting story, one that celebrates adventure, freedom, and friendship. This what makes the Straw Hat Pirates such a great found family.