human anatomy - Why do we have two of some organs, but not all?

We have two eyes, but we don't have two hearts. Why do humans have two of some organs, but not all?

Answer

Here is my overly succinct answer.

I doubt we will ever know this for sure. But, it basically comes down to ancestral bilateral symmetry in development; this defaults to two organs and is broadly symmetrical except where the organs are central. See this article for a more thorough answer regarding bilateral symmetry.

Further exceptions to symmetry occur where evolution pressured the body into not bothering to grow the second of an organ due to a waste of resources for the body, or functional advantages emerged from asymmetrical evolution. I always like the ears of an owl for an excellent example of two asymmetrical features that provided a discrete advantage when it comes to locating the source of a sound.