In the beginning of our Pathfinder campaign set in Eberron, I used two gimmicks to bring the party together:
- An afternoon train wreck with a zombie attack that night, forcing everyone (players and NPCs) to work together to search for survivors and then defend themselves against the low-level necromancer. I required that each player have some backstory and a reason for being on the train.
- During the disaster, somebody kidnapped the in-game NPC sibling of one of the players. The NPCs were busy and weren't willing to search for the sibling, so the players banded together to search for the sibling for the next several sessions.
15–20 sessions later, the players feel that their characters lack sufficient reason to work together, other than "because we have been working together". They've said they wished they'd put together a better backstory at character creation. As an adventuring party, they do quite well together, and there are no major party conflicts (e.g. different alignments or rivalries). The problem is that if they went back to a large town/city, their characters would be tempted to just split off and do their own thing.
In short, the players want their characters to be more bonded, but the characters lack sufficient back-story reasons to do so.
How can I encourage the party to band together more strongly at this point in the campaign?