96 of Shakespeare’s 154 sonnets have lines that do not rhyme. Why is that? To explain this we must first look to one of the most important events in the history of the English language, or rather þe Englissh tonge [sic].
Starting in the late 1400’s, there were many spelling debates about the pronunciation of certain words in the English language. Was knight to be read with a hard ‘k’ or a silent one? Did correction have a ‘c’ instead of a ‘t’?
The result of all of this was the Great Vowel Shift, which by late 1600’s had turned ‘i:’ into ‘iə:’, ‘e:’ to ’i:’ and ‘o:’ to ‘u:’. Err rhymed with her, one with alone and reason with raisin. Should did not rhyme with wood as should kept its ‘u’. Play, prey and sea all rhymed. So did sea and thee – or did they?
Shakespeare lived in a time when English underwent significant changes; however, these changes didn’t happen in a single day. So yes, sea did in fact rhyme with both play and thee at the same time as it was in the middle of a vowel change named the “meet-meat merger”.
This can be understood more clearly by going to a previous example. Remember that reason rhymed with raisin?
If reasons were as plentiful as blackberries,
I would give no man a reason upon compulsion.
– Henry IV part 1 act 2
Shakespeare’s rhymes and wordplays never went anywhere, when read with the accent of the time they are still clear as the summer sky. It’s just that English has changed.