There are several nouns in the English language that are always plural. It’s one of those odd quirks that you don’t really realise until it is pointed out to you, at which point it seems exceedingly puzzling. These nouns are pluralia tantum, “plural only”, and used for “a noun which is used only in plural form, or which is used only in plural form in a particular sense or senses.”
Over at Oxford Dictionaries Blog, they have a list of 12 nouns that are always plural, along with explanations to accompany them. These nouns are: trousers, scissors, glasses, clothes, marginalia, folk, shenanigans, loggerheads, cahoots, amends, smithereens, and thanks.
We use scissors (and pliers, and forceps, and tongs, and tweezers…), but we never use a singular scissor. We have the word scissor, used in phrases such as “scissor-kick”, but for the actual cutting object, we use the plural form.
We wear trousers (pants, leggings, jeans, tights, etc…), but never a singular trouser. Clothe is not the singular of the word clothes, but means something else. Glasses, goggles, spectacles, binoculars, all are always plural.
You are exceedingly unlikely to go on a single shenanigan, or encounter a single loggerhead. You can be at loggerheads with some folk over which shenanigans you want to partake in though.
It’s rather fascinating discovering these pluralia tantum. Have a single thank for it. Onwards!