When water is mixed with other substances it takes on the properties of those substances and is thus no longer fit to be used for purification. When it is mixed with impure substances it becomes impure. Likewise we must be careful who we mix with so that we do not take on negative attributes and lose our purity.
A small amount of water easily becomes impure, but when more water is added to it until it becomes a large amount, it is now pure and can be used for purification. For that reason we are encouraged to come together for worship. When there are a large number of us our impurities are diluted until they are no longer manifest.
Extracts from Sayyidi Habib Umar bin Hafiz ‘s classes on the rulings and etiquettes of purification from Habib Abdullah bin Husayn Balfaqih’s “Kifayat al-Raghib” in Shafi`i fiqh.