Spokes don’t break from overloading, but from fatigue as the spoke is loaded and unloaded when the wheel rotates. The wheel flattens at the bottom, which unloads the spoke. With each wheel revolution, every spoke is slightly detensioned, and then tensioned again. Over time, that causes the spoke to fatigue.
To get the maximum life out of your spokes, you want the detensioning to be as small as possible. That is what double-butted spokes are for: They are thinner in the middle, so they can stretch more, which means that they don’t detension as much as a thicker spoke would. Yet the ends, where spokes break, are thick and thus will last a long time. It may seem counterintuitive at first, but the thinner mid-sections make double-butted spokes more durable than thicker straight-gauge spokes.
Wheels tend to go out of true when you hit a bump and a spoke detensions so much that it goes slack. Now the spoke nipple unwinds as the spoke is tensioned again… and the spoke now has less tension, so it will go slack more often, allowing the nipple to unwind more and more.
For more information about wheel science and wheel building, we recommend the late Jobst Brandt’s excellent book The Bicycle Wheel.