Some siblings are inseparable, talking daily and sharing even the most TMI dating updates. Other siblings don’t get along at all and are, in some cases, completely estranged. “Siblings by chance, best friends by choice,” the cliché goes. But ask real people about their own family dynamics, and their reactions will probably vary wildly.

Even if nothing went wrong, “it’s normal for brothers and sisters to be more like friendly acquaintances who happen to share overlapping lives,” shares Erin Runt, a Chicago-based licensed therapist. “There’s this idea that how frequently you’re in contact represents how emotionally close you are.” Realistically, however, that isn’t the case for many people (despite what family sitcoms or your friends’ picture-perfect Instagram stories might suggest) — and it’s nothing to feel guilty about either.

So what makes some siblings best friends while siblings don’t get along? Aside from a major conflict, here are a few factors to consider, according to family therapists.

1. Parental involvement

In some households or cultures, parents are intentional about raising their kids to be close: to look out for and take care of each other, to see each other as built-in allies from a young age. Maybe your mum pushed you to include your younger sister during playtime, or your older brother was the one who taught you how to swim, ride a bike, and drive. According to Erin Runt, early experiences of support and collaboration can quietly shape how siblings relate to each other as adults.

2. Favouritism in the family

Just as positive experiences can bring siblings closer, negative ones can mean that siblings don’t get along. “Favouritism is one of the many things parents do, even if it’s unintentional,” says Karen Gail Lewis, a therapist and author of Sibling Therapy: The Ghosts that Haunt Your Client’s Love and Work. It doesn’t matter whether you’re older or younger: the one who didn’t get “special treatment” — whether that meant less leniency, more criticism, or constant comparisons — might hold onto resentment, which can make it harder to build a positive, trusted relationship over time.

3. Shared life experiences

Even if siblings didn’t get along as kids or teens, Runt says they’re more likely to bond later if they hit the same milestones. “Maybe they both started having children or got married around a similar time,” she says — changes that may naturally lead them to swap advice or lean on each other for help. Ultimately, these are experiences that can create fresh common ground and make the bond feel more organic, not forced.

4. The size of an age gap

While people love to speculate the ‘ideal’ gap between siblings, “there’s no clear pattern that predicts closeness in terms of age or sex or gender,” Gail Lewis says. In some instances, being six or more years apart can make it harder to relate to each other on a peer level, whereas growing up on a similar timeline can create a more friend-like connection.

That said, Gail Lewis points out that the reverse can be true as well: “If you’re too far apart in age, the good thing is it removes all competition,” she says. Meanwhile, those closer in age may be more likely to feel compared or subtly pitted against each other, which can strain the relationship.

5. Personality differences

Sometimes, the reason you and your siblings don’t get along is as simple as having little in common. And according to both therapists, major personality differences can be so pronounced that, if it weren’t for living under the same roof, you might never have chosen each other. It can be difficult for an introverted homebody and a complete extrovert to want to do the same things, just like a rebellious risk-taker who’s always getting into trouble may not naturally befriend a cautious rule-follower who avoids attention.

Ultimately, not every sibling bond has to look like the Gellers’ from Friends. What matters is finding a dynamic that works for both of you — whether that looks like confiding in each other every NSFW life update, keeping things cordial and only catching up during holidays, or doing the occasional FaceTime to complain about mum and dad — and considering that your own form of bonding.

A version of this article was originally published in SELF.