Ordinary nouns have a singular (one parakeet) and a plural form (two parakeets or even a pair of parakeets). Don’t bring up the joke about a “pair of keets,” please.
Many items come in pairs: earrings, shoes, socks, etc. While we would rarely wear only one shoe, we can speak of one shoe or a pair of them.
But a small set of nouns never occurs in the singular: a pair of scissors (you’ve never heard of “one scissor,” have you?); a pair of pants, boxers, shorts, etc.
If a pair of something is two of them, why doesn’t a pair of tweezers come from taking one tweezer and matching it with another tweezer? Have you ever see one pant by itself? Or a single plaid boxer?
If it must be a pair of pants because there are two legs (even if they aren’t each individually called a pant), then why don’t we speak of a pair of shirts? I mean, there are two arms, so what’s the difference?
This just goes to show that words don’t always mean what we think they do, and language is messy. It follows rules and then flouts them and allows for quirky variations.
(check out Chris Levine’s blog Technological Information, where the hilarious “Rock, Paper, Scissors, Lizard, Spock” image came from!)