Great Vowel Shift
A major factor separating Middle English from Modern English is known as the Great Vowel Shift, a radical change in pronunciation during the 15th, 16th and 17th Century, as a result of which long vowel sounds began to be made higher and further forward in the mouth (short vowel sounds were largely unchanged).
The causes of the shift are still highly debated,which required a different kind of pronunciation.
In Middle English (for instance in the time of Chaucer), the long vowels were generally pronounced very much like the Latin-derived Romance languages of Europe