And the other characters of the party?
- It's assumed that after the party has been adventuring together awhile (i.e. long enough to reach a high enough level where this discussion matters) there is significant camaraderie between them. What is personal to one is of at least some importance to the others.
- Someone asking for help is a standard adventure hook. There's no reason why that someone can't be someone in the party. Seriously, how many entry level adventures basically boil down to "run this errand for a high level NPC"? Why can't the one of the PCs be that person now that they're of significant level?
- Another underlying assumption is that the heroes are actively looking for adventure and want to work together. If a character is more on the mercenary side, they can be motivated by promises of payment, glory, and treasure, just like any other adventure. OTOH, if a player chooses to actively ignore problems that aren't directly related to them, they're going to have trouble with D+D in general.
- Don't ignore the metagame. After an adventure where the party saved the world from hordes of undead and the cleric got a chance to shine, it's a nice change of pace to focus on the thief for a session or two.