Creating a living world, no fed-ex quests

Random gaming thoughts:

I'm a big fan of running games that I make up whole-cloth. No published adventure for me anymore, thanks. I enjoy the freedom that it gives me in order to give back to my players. I can make a game that revolves around and is based on the characters, not one in which they're somewhat-interactive spectators.

One of the difficulties that I've run into with this approach is creating a living world. I am fine with creating the good guys and the bad guys, and all of the folks in between, and even giving them motivations and goals and things like that, but what I sometimes run into difficult with is coming up with Quests To Give To The Players. I should pause - that's not my actual verbatim approach, but kinda defines what I'm running into.

No matter how fun the game is, the 10th time that you have to Go Do Something For Some Guy So That He'll Give You What You Wanted In The First Place, it starts to feel repetitive. Sure, there's tons of variety, go Get This, go Deliver This, go Kill This Guy, go Mediate This Dispute, go Find This Thing, go Ask These Questions, go Snoop Around This Place. I guess it feels like I've fallen into a rut of viewing the characters as errand-runners for the people in the game.

In what ways can important and semi-important NPCs interact with the characters, in ways that are not so contrived? Let me pause again, I feel kinda like I'm asking "Okay, but how do I build a house?" which is not a question with a quick, simple answer.

I've even spared a few minutes to think about how things work in the "real world". Granted, there aren't as many dragon-hunters, or vampires or ghost-catchers in the real world, but I was trying to quantify how interactions work. And maybe, again, I'm taking the wrong approach. My mind now jumps to movies like Lock-Stock and Two Smoking Barrels, which is, I suppose, an infinitely twisted series of "fed-ex quests". Perhaps a small part of the difference is that instead of entirely being "Mr. Johnson tells to you run out and fetch him the such and such", the characters there were sometimes placing themselves into a predicament, and though asked for particular results, the motivation was different (I'm thinking of the gambling debt).

Anyway, there's my thoughts. Suggestions?