May was very pretty .<br>You know sometimes , when you are used to a pretty face , how , when it comes into contact and comparison with another pretty face , it seems for the moment to be homely and faded , and hardly to deserve the high opinion you have had of it .
May was only pretty when not being compared.
The statement makes it seem like when May is compared to another pretty face, it seems to be homely and faded. But, it was talking about pretty faces in general, not just May's. 
( z1 pretty :mod ( z2 only ) :time ( z3 compare :polarity - ) :domain ( z4 date-entity :month 5 ) )