Loads of reasons. Subjects, objects, actors and the positions of words in spanish. Add to that the fact that accents have a staggering variety in the English language. Travel 100 miles in england and that accent changes completely. I understand that english films that get shown in the US sometimes need subtitles, due to "incomprehensible regional accents"Vowel sounds in spanish are consistent, although there are different sounds for certain dipthongs, and flexibility around certain consonants. Vowel sounds in english change have a massive variety. Even fluent english speakers will often struggle to understand someone speaking perfect english with an indian accent. Just be patient and smile. Repeat slowly and clearly, try not to get frustrated. And learn spanish! The more you learn the more you realise quite how different the two languages are, despite the common root.