Why do opposites attract?

• forces between electrical charges
• force with positive and negative charges that attract
• Like charges repel
• Why we say that the things that attract electrically have charges with opposite signs, positive and negative.
• Look at a few things, you will find they can be grouped into three batches. Any two copies of some object ("likes") fall into the same one of these 3 batches.
  
  1. Ones that don't have any electrical forces with any others. We call these electrically neutral, and say their electrical charge is zero, for obvious reasons.
  2. A batch which all repel each other.
  3. Another batch which all repel each other.
  All the objects in the second batch attract all the objects in the third batch and vice-versa.
  
  Why do we say that batches 2 and 3 are "opposites"? If you take any object from batch 2, you can get some object from batch 3 which just cancels its electrical charge, meaning that the combination is neutral. When there are two things which add up to zero, that's just like positive and negative numbers. So we say that every electrical charge is either a positive or a negative. Which batch we call positive and which one we call negative doesn't matter, so long as we don't keep switching the names around. As it happens, we call electrons negative and atomic nuclei positive. Whether some big object is positive or negative depends on whether it has too few or too many electrons to cancel the charge of its nuclei.