The bane of every learner of English (native speakers included) is the vowel. English vowels can take on almost any value. For example, “ou” in the following words is pronounced in different ways:
(More examples of “ou” could be given, I am sure – leave them in the comments if you like.)
Why are these words pronounced the way they are? BECAUSE REASONS! THAT’S WHY! In so many words, that’s what is told to those who would learn to speak English.
Well, if you have mastered English pronunciation in the year MMXIV then go back with me to the year MDCXCIII where you would have a hard time recognizing many of the words you’ve worked so hard to learn. In this case, we are only examining dipthongs, i.e., double vowels (like “ou” above). Try the test at the end, and prepare for failure. Like I said, abandon all hope ye who enter here!
You gave up. I knew it.