Friendship hardly seems like something that needs worldbuilding. It's a basic human behavior, right? We all make friends?
Sure -- but what friendship
means does not stay the same.
Starting at: Who can you be friends with? Then and now, social divisions may complicate the answer to that. Can men and women be friends? If sex segregation means that women aren't supposed to go out into society or interact with men who aren't their relatives, then cross-gender friendship is pretty much restricted to a trusted cousin or two. (Even then, the relationship is likely to be spoken of in familial terms instead.) But a more egalitarian society may still be dubious of friendships between men and women, with many people assuming there will always, inevitably, be an undercurrent of sexual tension there: friendship as a consolation prize, or a barrier to head off escalation to something more.
What about friendship across class lines? That will often be awkward, even without formal hierarchies of status to get in the way; after all, if one person's struggling to make rent and the other could buy their entire apartment building, you have some inherent inequality there. This gets particularly thorny when one person employs the other: however well they get along and enjoy each other's company, their personal and their business relationships may wind up pulling in opposite directions, to the detriment of both bonds. In that light, it's not surprising that many past societies would have said straight-out that such connections cannot be true friendship. That can only exist between equals.
Class also shares a quality with racial boundaries, which is that both of them are deeply interwoven with culture. People from different groups may have any number of cultural differences, creating significant contrasts in how they spend their free time, what they eat, and even how they converse. These things don't
prevent friendship -- we have far too many real-world examples proving otherwise -- but they can make it more difficult, with opportunities arising for misunderstanding or conflict.
But what does it mean to be friends, anyway? So far we've been glossing over that as if it can be taken for granted . . . but one look at an elementary school (where kids are very much learning the social ropes) shows that's not the case.
The answer here isn't just cultural but personal, too. One individual may refer to anybody they know in a positive, non-business capacity as their friend; to their neighbor, most of those people are "acquaintances" or "people they know," with the term "friend" reserved for those who enjoy a deeper connection. Digital relationships particularly complicate this, with the rhetoric of "friending" someone on a social media network implying more connection than actually exists. And how many friends can you have? Most people don't put a real cap on that, but they
may feel you can have only one best friend at a time, and that to throw the superlative around more broadly cheapens its meaning.
Part of what muddies the waters here is that we rarely have formal markers for friendship, the way we have them for marriage. Friendship bracelets (which are said to have historical origins in Central America) started being shared in the '70s or '80s; however, they're not universally used, and people can wear that style of bracelet without it signifying anything in particular. Children may declare "you're my friend now" or ask "are we friends?", but adults -- at least in the societies I know -- are more likely to leave it implicit, with all the social pitfalls that entails.
Because part of friendship is being able to share certain intimacies with the other person. That might mean dumping your troubles on them, knowing (or at least having good reason to hope) you'll receive a sympathetic hearing; it might mean asking them to do things for you, without needing to negotiate some kind of explicit compensation or trade. If you try either of those things with someone you assume is a good enough friend for it, only to find they don't see the two of you as being that close . . . oof. It can get very awkward, very fast.
And "intimacy" may go a lot farther than that. In much of the past, and in many parts of the world today, it's entirely normal for friends to show a degree of physical affection that my fellow Americans generally reserve for significant others: hugging is okay, at least for some people in some circumstances, but holding hands as you walk down the street? Kissing, on the cheek or on the lips? Taking a bath together, or sharing a bed? Those things look romantic to us, not platonic.
The same goes for emotional intimacy, or rather, how it's expressed. If you read the letters of same-sex English friends from the nineteenth century, they regularly speak of each other in terms so passionate, you could easily mistake them for lovers. And in some cases, we have reason to surmise that's one hundred percent true; deep friendships could indeed be a cover for a type of relationship not sanctioned by society at the time. But that cover worked because friends
did write to each other in such terms, without anybody assuming that "I long to kiss your lips again" carried sexual implications.
Which makes for interesting challenges when it comes to fiction. If you write such behavior into your invented society, then it's likely that a
high percentage of your readers are going to interpret that as shippy. In some ways that's fine -- a certain type of reader will ship all kinds of pairings you never intended -- but in other cases, that may make your audience think you're queer-baiting them, suggesting something and then not delivering. Even if they don't feel cheated, the weight of association is going to shift how they read the characters' behavior, adding sexual overtones where none were supposed to be.
Finally, there's the question of how friendship
ends. Again, children tend to make it more explicit: "I'm not going to be your friend anymore!" Social media gives us the passive-aggressive option of unfollowing somebody, which they may or may not even notice happening. If you have some of their belongings, or they have a key to your place, a sufficiently bad rift may entail a dramatic scene of shoving somebody's stuff back at them or revoking their access. But mostly we just drift away, ending the relationship as ambiguously as we began it. . . with every bit as much room for uncertainty and misinterpretation.
Seen in that light, there's frankly a lot to be said for worldbuilding more overt structures around the beginning, ending, and depth of friendship between your characters. Or maybe not: maybe crossed wires and hurt feelings are exactly what your story needs!

(originally posted at Swan Tower:
https://is.gd/QcgTOl)