Sometimes players of adventure games stop and ask, "Wait, why am I risking my life in this dangerous adventure?" They balk at entering the dungeon, braving the Weird Woods, or seeking the dragon's hoard.
"Why would I ever do that?" somebody asks.
Referees, you should beat them to the punch. You ask them this question.
I have been known to go around the table and ask each player at the beginning of a session, "Why are you on this adventure?" And they understand that they need to motivate their PCs.
I get answers like, "I'm searching for a magic object--let me come up with the details later."
"Okay," I say.
Another: "I want to prove to my village that I'm better than all of them after they picked on me! I'm going to get rich and powerful. Eventually, I'll return home and I'll be the boss."
"Okay," I say.
"My father was a treasure-hunter and I want to make him proud."
"Okay," I say. "Think about the details of this and tell us later."
And each player in turn does a pretty good job of coming up with motivations. Sometimes they make up shared motivations that they can use together.
It doesn't take long. Then we play. I make the players do that self-motivation work because I have done a lot more to prepare, putting together a playground for them to explore. It's the least they can do to meet me part-way.
They may find new motivations as the game rolls along, too.
If they don't have a motivation and seem averse to the idea, I say, "Make another character who has a motivation to go on this adventure. You need a character with a goal."
"What if I complete my character's goal?"
"Then you find a new goal or make a new character with enough of a motivation to continue going on adventures."
The suggestion that they should make a new character when they already have one they like is often enough motivation for the player to invent a motivation for the current character.
Sometimes they have trouble coming up with a motivation. That's easy to fix, too. If they are stuck, just give them a hardship, or assign another player to invent one for the one who is stuck. Tell them they have a huge debt (currently quite a popular method), or that they are being pursued by some terrible evil, or invent a loved one in distress that they need to save. Make up something horrible that happened to their PCs, maybe something for which only revenge will give them peace of mind. Make up a wonderful object and tell them they used to have it, but now it's stolen and only this adventure will turn up the clues to find it.
Give 'em hell if they can't come up with their own reasons to be there. Pose threats. There's nothing like fear of suffering to motivate action.
In the end, though, you don't need to do this for your players. It's their job. Referees, you have enough to do.
Players, motivating your own character is part of player skill. Make the game more fun for everybody by including sufficient motivation with your character to play the game. Why else are you at the table?