The aligned, planarity of the planets, asteroids, rings, etc are just a side-effect of stellar evolution.

1. During the early formation of a solar system, a collection of material isolates itself, which each cohesive mass in a completely random orbit around the gravitational centre of the mass.
2. Whenever two such masses collide, if their masses collect together, or one mass picks up a large amount of the mass of the other, the resulting motion of the larger object will be be governed by the overall angular momentum. Roughly speaking this will average out the direction of motion.
3. As energy is lost from these initial collisions (absorbed in the material itself) most of the material, will tend to veer towards the centre. Most of this mass will become the "sun" for this solar system.
4. The remaining material, so long as it is not concentric with the remaining material, will eventually collide with other objects, and thus cannot be part of a stable solar system. But each collision reduces the number of "non-concentric" objects.
5. According to Kepler's first two laws (1) (2), the satellites of the sun will tend towards elliptical orbits. However, if they are not planar, they will tend to be pulled away from the centre almost randomly which will induce more collisions. So collisions, will both continue over time, and tend to reduce the number of objects over time until all objects are planar.
6. The solar system will become stable as soon as the objects find a set of orbits that all basically miss each other, and maintain this over the long term. Because of the previous conditions, this will occur when most of the objects are planar, in orbit in the same direction, and basically concentric (their elliptical paths do not cross.)
   

It is not required that the solar system converge to a simple stable system of concentric orbiting planets. Our own solar system has numerous exceptions:

1. Our solar system has three asteroid regions. The asteroid belt between Mars and Jupiter are made from many asteroids, roughly in a synchronized orbital path. The Kuyper belt is mostly beyond the orbit of Neptune. The Oort cloud is a group of asteroids very far out, and contains objects that are not really on the ecliptic plane. The first two have simply achieved converged motion that have very few collisions over time without having accumulated into larger planets. The Oort cloud, since it is so far out, actually has had too little time relative to the slow movement and low density of the objects to have settled via a sufficient number of collisions to have become planar. In fact Oort cloud objects on sufficiently eccentric orbits are a basic hazard of the solar system -- they are represented by random asteroids that enter the solar system and have the potential of colliding with the planets and moons on the inner solar system. One needs only look at the moon's craters to see how often this has occurred over the lifetime of the solar system. Other seemingly random objects entering the inner solar system from the Kuyper belt and Oort cloud include comets.
2. Many planets have satellites or rings (which are like mini asteroid belts) whose orbits are close enough to them that they are their own mini-system that ignores the macro-scopic solar system rules. But because they were involved in earlier collisions they will tend to be somewhat on the ecliptic plane.
3. Outer planets are less likely to collide with other objects because the density of the mass decreases the further away from the sun they are. For this reason, they will tend to satisfy the conditions less strictly. Hence Kuyper belt objects like Pluto approach the sun both closer and further out than Neptune, but are also have a very deviated angle from the ecliptic plane. Pluto's orbit misses Neptune (even though it looks like their paths cross if viewed from a perpendicular to the ecliptic plane) precisely because its orbital plane is lifted away from Neptune's ecliptic plane where their distances to the sun cross.