When objects (stars or planets) condense out of a nebula, they often form in pairs. For example, most stars are members of binary or multiple star systems. Our sun may be solitary, but in the galaxy, that seems to be an exception. This seems to be a natural consequence of how solar systems form. Of course, the boundary between star and planet is not clear...

Within 5 parsecs (16.3 ly) of the sun there are 53 known multiple systems, including 66 stars, 9 brown dwarfs, 13 extrasolar planets plus 8 planets in our system, most of them with at least one major satellite. The nearest star, Proxima, orbits a binary, Alpha Centauri. Our nearest neighbor is a triple system. Clearly, the star and planet-forming process is hierarchical. Pairs are often orbited by other stars, or even other pairs, and higher order multiple systems are not uncommon.

In additions, companions can be ejected, or captured, or formed as the result of collisions (as current theory about our own moon suspects).
Our knowledge of extrasolar systems seems to suggest that the smooth, orderly layout of our own solar system, with fairly circular, non-intersecting orbits, is the exception, not the rule. Maybe the older systems wind up that way, but they may be pretty chaotic in their youth.

Planetary formation appears to be a turbulent and unruly process.