Poor Jacob just can't get by without more changes coming at him every single chapter! This time around, the pack he didn't even want to start is growing with the addition Leah. With the exception of maybe Paul, Leah is his least favorite wolf.
Key plot points from The Twilight Saga: The Official Illustrated Guide
It's completely understandable how Leah would make the choice to join up with Jacob. She gets to stay with her brother and keep an eye on him while escaping from the pain she has to endure as a part of Sam's pack. She feels unwanted by the very person she loved more than anyone else. That is a special kind of pain. She's constantly subjected to Sam's guilt when he sees her. She's dealing with both his pain and hers.
It's also highly understandable that Jacob doesn't want to lead a pack. He didn't go off on his own because he wanted to be the Alpha. He was just able to use his birthright to break from the pack in order to take control of the situation and protect Bella. The addition of Seth and Leah forces him to become an Alpha in daily practice. That's not something he was ready for.
The shared concern of Bella is causing Jacob to form a reluctant bond with Carlisle and Edward. They understood the sacrifices he's made to be there and appreciate that. They all want Bella safe more than anything. Jacob isn't letting that stop his less than positive thoughts about the situation which turns out to be the inspiration for Edward's newest idea to help Bella.