Hawke picks up companions like he picks up everything else he comes across (sadly Sunshine hasn't divulged whether or not Hawke has always been like that and Leandra is gone now, Varric never knowing her well enough to ask prior to her death.) There's no rhyme or reason to it, he just finds them as he goes, one by one welcoming them into his life no matter how different they are to him. Varric was the first, he was there to watch them arrive one by one: the possessed Apostate former Grey Warden, the Tevinter fugitive, the Dalish pariah, pirate captain, the Guard-Captain, the exiled prince. They're all welcome in his home, he brings them gifts, he helps them with their own personal matters, he values their opinions. Hawke doesn't cast them aside. He keeps them close through the years the same way the chest at Gamlen's in Lowtown and later the one at the estate is full to bursting with every sword, every shield, every stave, every moth-eaten and Maker knows what else Hawke has ever come across.
It isn't always an honour to hanging around with Hawke. The hours are awful (middle of the night running around Kirkwall fighting gangs or sneaking around - they're not guards and hell guards have shifts), the pay varies (really, there's no pay but they share out what they find and it's been a long time since most of them have had to buy weapons) and they take on things none of them would in the course of their normal lives (Qunari, demons, abominations and more) but their lives would be so dull without it. When he tells the tale of the Champion it's about Hawke, it's about all of them, about the choices that have been made, the journeys they've been on, everything they've experienced with plenty of embellishment (Ferelden pragmatism makes for dull stories, if Hawke got his way then no one would want to know about the grandiose and seemingly impossible feats he accomplished) because Hawke has a way of casting a particularly long shadow. But there are times when he likes to sit idly by the fire in his private rooms, quill brushing against his chin after a quiet day (and there are quiet days not that any other sane person would recognise them as quiet) with a pint, just thinking. (On the nights he's not playing cards with Fenris and the rest of the guys. Or when Isabela isn't handing him her latest friend fiction for his opinion, not that he really knows if the stories of Grey Warden stamina are true and that Hawke or Bethany would tell her if she asked.)
There are so many possibilities for all of them, even for him although a lifetime of having to deal with Bartrand without a story is too damn depressing to dwell on for long so he thinks about everyone else. The more he gets to know them along the way noting down how they react to certain places or situations, throwaway remarks, tone of voice, body language and even the things they don't say, the more he wonders just what shaped them, how a single change might have forever altered them. If he had the time - maybe he will now that Hawke has strolled (it wasn't a stroll, it was a purposeful limp with Dog at one side and Anders at the other, both of them exhausted and ready to collapse) into the sunset - he'd write about them. Because they all have stories that are more than worthy of being told but such is the life of a companion, never the focus, someone there when they're needed and a story would be flat without them but save brief moments, it's not about them. But looking at them, so many of them interlink. In another life Merrill might have known the Warden, if they'd been someone other than a human mage. Isabela had her tryst with that very Warden and her Antivan Crow. Anders definitely knew her too growing up in the same damn Circle tower in Kinloch Hold although it's not a place Anders likes to talk about too much. Varric can't exactly blame him, he's sick of Templars and Mages and their conflict but he can see how it shaped Anders. He didn't get the same chances Bethany did to be free from whatever doctrine they grow up with in those towers; hiding probably seemed preferable to never truly experiencing the outside world. Fenris, Andraste's tits Fenris could have been different. If he hadn't been a slave, if he hadn't been in Tevinter, if he hadn't suffered at the hands of that particular Magister.
It gets confusing after a while because there are so many ways their lives could have gone, a great spider web of connections and intersections. It's damn lucky (in a roundabout sort of way he imagines because it's not as if they've had great lives to boast of, at least not in recent history) they all managed to end up in Kirkwall as they did. As far as his own life goes, he was the one who heard whispers of a warrior with an Apostate sister, that the last of the Amells had returned to Kirkwall, a warrior who survived Ostagar to boot so he's the master of his own destiny at least as far as he can be. He chose a companion's life and now as the storyteller it suits him just fine.