(From the book Shamati by Kabbalist Y. Ashlag)
Joy is considered to be love, which is being. It resembles a man building himself a house, but neglecting to make hollows in the walls. It turns out that there is not a way for the man to enter the house. Therefore a hollow, a space must be made, so as to make an entrance to the house.
Therefore where there is love, there should also be fear. For that is the void, meaning that he should be afraid lest he’ll not be able to aim to bestow.
Thus, when both are present there is completeness. Otherwise they each would wish to discard the other. One should try to have both present. That is the issue that one needs love and fear.
Love is being, whereas fear is a hollow, a space and only the joining of the two make for a whole.
This is referred to as two legs. That only when one has both legs, one is able to walk.