When your tongue glides from one vowel to next, you will get a unique sound from this work. In many cases, it almost a dual sound. Diphthongs are often difficult for English as a Second Language learners. This is most because this is a very unique rule within the English language subset.
What's actual makes this more difficult for new language learners is the existence of accents all across our country and the world, for that matter. Try to explain to someone how to say the word "tomato" or "water".