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Why do we call electrons 'negative' and protons 'positive'? It's an arbitrary convention but it's annoying: it means that when electrons flow through wires, the current is defined to flow in the OTHER DIRECTION.
If you ask people why electrons got called 'negative', you get a bunch of crap answers:
"Electrons are referred to as negative because of their behavior in an electric field. In an electric field, an electron will move from the negative pole to the positive pole, giving it a negative charge by convention."
Yes, but why THAT convention?
"If we re-designate all positive electric charges as negative and vice versa, while keeping their absolute value, the resulting physics would be the same. So exact choice is merely a matter of convention."
Yeah, it's a convention - but why THAT convention?
"In quantum theory of elementary particles (in a sense of irreducible representation of Poincare group with mass m and spin (helicity) s) if some operator Q of internal symmetry commutes (like electric charge charge) with Hamiltonian H...."
Irrelevant crap which never leads up to an answer.
The answer is that Benjamin Franklin chose this convention and nobody knows why.
In Franklin's day there was a 'two-fluid' theory of electricity saying electricity comes in two kinds. Around 1750 he developed a 'one-fluid' theory after showing that a rubbed glass receives an equal but opposite charge as the cloth used to rub the glass. He decided that electrical fluid was going into the glass. So he said the charge of the glass was 'positive' and the cloth was 'negative'.
Unfortunately it turns out that that electrons are going into the cloth.
The whole history is interesting:
@johncarlosbaez It could have been worse. Imagine if a Frenchman, say, had been working on the same things at the same time, only with the opposite convention, and French physicists then stubbornly stuck with it to this day while everybody else followed Franklin.
@mansr
The ones stubbornly using weird units are the people in the US, not the French 
@johncarlosbaez
@j_bertolotti @mansr @johncarlosbaez
The French also use a crazy system. The base unit of distance is metres (Earth's circumference/40,000 to make it roughly equivalent to a yard) but don't lead directly to volume in litres, you have to remember to do your measurements in decimetres. The unit of weight isn't the gramme but the thousand-gramme (kilogramme). Instead of a cube of sides 1 metre containing one litre and that volume of water weighing 1 gramme, it weighs 1 thousand thousand-grammes. ;)
@geoffl @j_bertolotti @mansr @johncarlosbaez Wait what? Nobody I know uses decimeters in real life, and we use grams for small quantities (e.g. when cooking). Plus, decimeters and kilograms *are* standard units.
@Arcaik @j_bertolotti @mansr @johncarlosbaez
That's kind of the point; the current variant of metric system, MKS, and the units people use, like litres, are not coherent (other metric variants include CMS and MTS). And while gramme is a unit of weight it is not a "base unit" so can't be used in other calculations, such as force = mass * acceleration.
"A coherent derived unit, for a chosen set of base units, is a product of powers of base units, with the proportionality factor being one."
@geoffl @Arcaik @j_bertolotti @johncarlosbaez I think it's fine to use a power of 10 (or whatever radix your number system has) scaling factor for convenience, at least in casual situations.