What about simple friendship?
We squeal when a group of characters proclaims that "we're more than that — we're family". It's a popular trope for a reason: it shows a deep bond between individuals who are not tied together by blood or legal papers, a bond based on trust and care. A bond that tells us that we are allowed to care for whomever we want, not just those whom we are forced to love because of familial relations.
Anime and manga creators adore the found family trope, throwing together absolutely different characters and making them play house or just travel together on their huge quests. It stems from a generation filled with dysfunctional families and serves as an escapism mechanism, allowing people to believe that even if you were raised somewhere where you weren't needed, you will be able to find those who will love you as unconditionally as someone related to you by blood should. And this is a nice thought, a very uplifting one.
But why should it be a family? Why should the bond transcend friendship to be considered wholesome? And why is familial bond the strongest one you can have?
Let's look at Gintama, a show that has at least two found families: the Yorozuya trio and the Shinsengumi. Both technically fall under the umbrella term of a found family: the characters live under the same roof, they engage in similar activities, they are driven by the same goal, they participate in domestic shenanigans that occur due to the closeness, they eat food together, just like a family would. They have a history, both in their respective found families and with each other, and this history is exactly what creates this bond.
So if there's a history behind it, can this love actually be considered as unconditional as actual familial love? This trust is earned, this reliability has been proven, and this bond has been forged through activities.
Perhaps the difference between a strong friendship and a found family lies in forgiveness. If your friend pisses you off constantly, you'd cut them off, you don't need that negativity in your life. But with the person from your family, no matter whether it's the biological one or the found one, you'll bear it for a longer time. You'll let them go with so many things that you wouldn't allow your friend to do.
And here lies the terrifying part of the found family trope: it's easier to lose a friend than a brother, and it's easier to make a friend than a brother. And you can't say for sure that your found family won't be dysfunctional and won't bring you pain, but the bond is too strong to sever it on a whim. Just like with a biological family member.
And maybe that's why some found families should just stay friends.