Always bugs me seeing Chibnall fans use 12 and his relationships to justify how poorly written and developed 13 and her companions are, that because 13 is so closed off because of the losses 12 suffered. One of the key lessons of 12's run, and that era overall, was that losing people hurts, and it is something everyone will go though eventually, the Doctor was no different than anyone else, but you shouldn't push people away to avoid that pain.
"Times end, River, because they have to. Because there's no such thing as happy ever after. It's just a lie we tell ourselves because the truth is so hard."
"No, Doctor, you're wrong. Happy ever after doesn't mean forever. It just means time. A little time."
River and the Doctor didn't live happily ever after, but they were together for a time and they were happy, and the Doctor would've missed out on all that if they'd kept running away. The loss of River and Clara deeply hurt 12, but they were able to overcome it and forge new friendships, and even after everything that happened with Bill, Missy and Nardole, when they were on the verge of committing suicide, they found the strength to keep going, to become someone new, but not before passing on some hard won wisdom.
"Never be cruel, never be cowardly, and never, ever eat pears! Remember, hate is always foolish, and love is always wise. Always try to be nice, but never fail to be kind."
Unfortunately, it feels like Chibnall only heard "Laugh hard, run fast" part of that speech and not the " be kind" part. 13 was inconsistently closed off solely for lazy drama that never went anywhere and never resulted in any kind of substantial change for her character or anyone else's or any sense that 13 truly understood how damaging those actions were. It was just sloppy, careless character writing that wasn't informed by what the previous Doctor went through and didn't really understand their story, just knew some basic facts.