Great Vowel Shift
During the period of Middle English, there were five short and seven long vowels. Through the GVS, the long vowels /i:/ and /u:/ gained counterparts they did not have before, restoring the balance in the vowel system. This attempt to achieve balance is thought to be one of the reasons for the start of the GVS. Along with other theories, like the pull-chain and the push-chain theories.