Translation:Ο χυμός μου.
It just does ;-)
To cut a long story short, Greek uses articles an awful lot more than English. As a very rough rule of thumb (not always true but useful when you're starting out) the subject of the sentence will require an article, either definite or indefinite. Objects, it's a little more complicated, but don't worry about that just yet.
You often use them in ways you wouldn't in English - eg:
- Η Αυστραλία είναι μεγάλη χωρα - lit 'the Australia is big country'
- Ο Γιαννής είναι καλός άντρας - lit 'the John is nice man'
- Το κουνέλι τρέχει γρήγορα - lit 'the rabbit runs quickly'
You'll pretty quickly get the hang of it; I initially thought it was really odd, but it comes completely naturally to me now.