You know, back in the day we never seemed to feel the need to come up with intricate backstories for our characters, or motivations, or any of that.
We rolled up a character, maybe had an idea in mind from the latest fantasy book, movie or video game we'd accessed, and were ready to explore some dungeons, kill some monsters, and loot their treasures.
Then somewhere along the way (around 97 for me, when I fell in with the Evansville group and I was chock full of ideas for stories that I'd never get around to writing) I began to want those backstories and motivations. Maybe it was something about the 2E books, which I picked up around then. Maybe I felt like it was more 'mature' that way. Maybe it was just something in the air.
For the past few years, though, I've been worried less about backstory or motivation. If I've got some ideas, great. I'll roll with them. If my players want to come up with them, again, great.
But I find D&D works best if the only real motivation your characters need follows this statement:
I boldly venture forth into the dark places of the earth, face countless horrors, and all for fortune and glory.
Anything more than that is unnecessary, but the above is vital. If you're not out for the gold and glory, you probably shouldn't be playing in my D&D games (other games will vary).
[And Dave, if you're reading this, this is just a general spouting off of ideas, not directed at anyone in particular in our group.]