When you're in your 20s, most friendship groups form because of some uniting cause. You're all single, you all like tequila and sequins (in that order), or you all memorised Roald Dahl poems as a kid.
Whatever it is, you're all in the same situation, you can relate to each other in some way. You rely on each other, talk about your tumultuous dating lives and relationships, your fears about the future, or absolutely nothing at all.
Out of that, a sort of day to day dialogue emerges. You have a casual, almost unremarkable routine of checking in with each other, and popping round, and it's a sort of barometer: you all seem to know how everyone is.
But at some indeterminate point in your 30s, slowly that begins to change.
And you know it's inevitable, natural even: but that doesn't stop it feeling strange when your good friends no longer come to you with their day-to-day stuff.
More often than not, they've got someone at home who can listen and take the load off their mind midweek, rather than nipping round to yours, or for a quick one in the pub.
For me, this year's heartbreak hasn't been a romantic relationship dwindling. It was realising that little by little, my friends no longer need me in the same way they did before. And in turn realising that I, being single, still need them in the same way I always have.
Which was the thought process running through my mind a couple of weeks ago, when winter was in the air, and my mood was dogged down one day mid week.
My next planned social event - because now, they are almost always planned - was still a couple of days away, an impossible work situation was feeling impossible, and family matters felt like they mattered a lot.
What I really fancied was a quick chat, something to perk me up: but my last failed attempts at rousing the gang on Whatsapp for a spontaneous drink were ebbing at the confidence to make the same suggestion again.
I knew, if I was to send a message, or an email, or make a call and say I could do with a chat, that responses would come and someone would be there.
But when your mind is telling you that everyone else has somewhere else they'd rather be, then the simple call or text that would fix things can be the hardest one to make.
So I didn't, and the day trundled on, along with my mood. Until just before the end of the day, when a text message arrived.
The name surprised me. It was a friend who had largely disappeared this year, leaving one of the biggest gaps of all. But there she was - as if she knew I needed her - with just two words:
And that was all it took. We chatted away, she asked how I was, and we arranged a Sunday afternoon out. Little by little as the inane back and forth continued, my brain lifted itself out of its slump.
It reminded me just how restorative a simple how are you can be. That friends are still there, but they need to know that you need them.
And single or not, I think that reminder works both ways.