We notice that Allah (swt) also uses the passive voice in this verse. He (swt) says before (and its fruit brought low within reach) while He could have said: ‘We brought its fruit low within reach’ and He (swt) also says (Passing around them are vessels of silver). Here Allah (swt) says:

(… وَيُسْقَوْنَ فِيهَا كَأْسًا) (They will be served therein with a cup …): He (swt) could have used the active form instead of the passive one, but Allah (swt) wants to say to man by whose name this Surah is titled: If you endure in this worldly life and you are patient at its suffering and hardships and the life is tiring for you because it confronts you for your patience in belief and obedience to Allah (swt), then in the Paradise the matter is completely different. Allah (swt) has subjected the Paradise for such believer to serve his desires even before he asks for them. Thus, do not be sad or worried, as the outcome is for the pious.          

(… وَيُسْقَوْنَ فِيهَا كَأْسًا) (They will be served therein with a cup …): The are served and do not prepare what they want by themselves. They are given the drink without any intervention from them. It is not important to know who give them the drink, rather to know what they drink. Thus, Allah (swt) uses the passive voice.

  (  كَانَ مِزَاجُهَا زَنْجَبِيلًا … ) (…whose flavor is Zanjabeel): The drink is mixed with Zanjabeel (ginger). In another verse, Allah (swt) says:

 إِنَّ الْأَبْرَارَ يَشْرَبُونَ مِن كَأْسٍ كَانَ مِزَاجُهَا كَافُورًا

(Indeed, the righteous will drink from a cup [of wine] whose mixture is of Kafur) (Al-Insan: 5)

So, the tastes and flavours are different even if the drink is the same.

(… مِزَاجُهَا… ) (…whose flavor …): This means that the drink is mixed with little ginger because everyone knows the bitterness of the ginger in this worldly life. However, the ginger of the Hereafter is different from that of the worldly life.