There are a lot of trees on both sides of the street=There are a lot of trees on__ __of the street.为什么只能填either而非each

There are a lot of trees on both sides of the street=There are a lot of trees on__ __of the street.
为什么只能填either而非each

either side
either 是“两者中的任何一” = 两者都 both

There are a lot of trees on both sides of the street=There are a lot of trees on each of the street.
either 是两者中的任何一,each是多者中的每一个，意思不一样的~

either side

either side
街的两边有一些树=街的每边都有些树