That is one jolly saint nick

Girl to the right of Grandpa Serial Killer

What if you were born into a Thornberries esque family, grew up traveling the world as your researcher and adventurer parents did their studies in the wilds, ancient ruins etc.

You fell in love with adventure and ancient mysteries, so after graduating from adventurer college (you were a legacy admission), you intern for [initial quest giver for campaign].

Easy one, done off the top of my head.

I'd argue that they can have equally good motivations, it's just harder to do. A lot harder.

sounds like a good background for a proper paladin, no?

person grows up well adjusted and happy, including being taught to have morals, so when they learn about the horrors in the world they go "fuck that noise" and pledge themselves to make the world less shit.

And since they're just.. a normal dude.. they can be the party's "straight man" who sees a half-feral goblin mage join the party and looks into the camera like in The Office.

I honestly feel that the motivations for someone from a happy home are often better. They are people who want to adventure for some very compelling reason (that you are forced to come up with). They are invested in the world, with things to loose, things they want to accomplish, loved ones to threaten, history and interests they want to share etc.