What was Big Willy Shakes going for?
Dromio of Syracuse is confused. He comes across S. Antipholus in the marketplace and immediately gets ribbed for doing nothing. He hasn't got a clue in the world why S. Antipholus would give him a hard time. Why?
Turns out the lost set of twins have been in Ephesus the whole time. Remember the boys also share names: Egeon's twin sons are both named Antipholus, and the twin servants are named Dromio. This is the part where all of the confusion begins.
Dromio of Ephesus shows up at the marketplace… and mix-ups ensue. Earlier, E. Dromio had been sent by E. Antipholus' wife to bring the tardy E. Antipholus home. E. Dromio mistakes S. Antipholus for his master, and begs him to come to dinner. Meanwhile, E. Antipholus's wife is so peeved he's late that she's been beating poor E. Dromio.
S. Antipholus gets testy, as he mistakes E. Dromio for his S. Dromio, and thinks this man is talking nonsense (especially as S. Antipholus has no wife). S. Antipholus asks about the 1,000 marks he gave S. Dromio to use to get a room at the Centaur (the inn). S. Antipholus figures that his servant is just messing with him.
Tensions get higher as E. Dromio keeps trying to get the wrong guy, S. Antipholus, to come home to E. Antipholus's wife at their house, the Phoenix. S. Antipholus, fed up, smacks poor E. Dromio, and E. Dromio runs off, confused and now beaten twice. Of course, S. Antipholus figures this is all just too weird to make sense, so he decides that S. Dromio was cheated of the money and didn't want to admit it.
Furthermore, S. Antipholus concludes that Ephesus is a crazy country, full of quacks and sorcerers. Satisfied with this perfectly reasonable explanation, S. Antipholus heads off to the Centaur to find S. Dromio and his money.
So when S. Antipholus sees S. Dromio and starts questioning him about something E. Dromio said, (not knowing it's not the same guy), everyone is confused. S. Dromio says there's no reason—none at all—why S. Antipholus should treat him this way, and he's right. That's just some of the craziness that happens when your evil twin is running around town and getting mistaken for you. Literally (well, except for the evil part).
Shakespeare uses this phrase one again in As You Like It, when no twins are confused. Still, some things just aren't explained all that easily. The Old Bard might have come across a similar phrase in The Boke of Nurture by John Russell (1460): "as for rhyme or reason, ye fore-wrighter was not to blame." Who can say for sure? Even if he did, he changed around the words to make the phrase we hear most often today.