I read a comment on someone's rant book stating that Jc wants to hold position as Kian's only bestfriend but is not trying hard enough. And I couldn't agree more.
Jc wants all the attention from Kian to be directed at him without giving any to Kian.
Which of course, won't fucking work.
Jc keeps getting jealous when Kian's attention is directed completely at someone else (e.g. Bobby) and not at Jc himself. He does a small thing to direct Kian's attention back at him and succeeds for a short few seconds - and after that, he does nothing that could catch Kian's attention again. He instead runs to his girlfriend and starts spending more time with her.
It's 'known' by now that Jc is still possibly unsure about his feelings considering Kian which is why he turns to Chelsey (to convince himself that he's straight).
I also read (credit to kljcrose for this theory) about Kian & Jc and their small pattern that started happening.
When Kian was with Meredith, Jc and him started vlogging and were rarely seen together. After K&M broke up, KnJ started filming videos again, but this time together.
They grew close again and were flirting.
And now we see the same situation with Jc and Chelsey. They started dating a few months ago, and at the beggining of their relationship, Jc was still filming with Kian. But now, few months into their relationship - Kian and Jc are rarely seen together. Kian started hanging with Bobby more.
Kian and Bobby are in good terms when Kian spends more time with Jc but are in great terms when Kian is spending less time with Jc. It's almost like Kian is only hanging with Bobby more only to show Jc that he 'doesn't need him'. That he has another best friend he'll spend more time with. I'm pretty sure Kian and Jc will grow close at some point (when Jc breaks up with Chelsey) and Bobby will be left behind again.
Also, to note what everyone noticed already, Kian is not following Chelsey nor is she following him - despite her following Franny, Corey and Bobby.