A mystery really motivates me such as what the OP mentioned in their post. Or like in BG2 when you wake up in Irenicus's dungeon and you're trying to work out who this guy is, what's happened, why you're locked up, what all these people in jars are doing etc. Then you finally get out of his hideout and then he disappears!
And then you get so much fun sidequests to do, that you forget about Imoen and Irenicus completely, until you recruit a new companion, and your PC says: "but you should know that my priority is to track down an evil wizard and rescue my friend!"... oh yeah, that was a thing. I supposed I had enough money to pay the thief guy like 10 hours ago. Whatever, what are other quests I can do...
At least, that was my first BG2 experience.
