Answer:
Sigmund Freud's definition that hate as an ego state that wishes to destroy the source of its unhappiness stressing that it was linked to the question of self-preservation. (The bold and italicized phrase is part of Sigmund Freud's full definition)
Explanation:
Shakespeare, in his play Romeo and Juliet, never tells us of any cause of the feud between the two families. However at present (during the play) the feud is ego-centered. Both families' egos make them keep hating each other. This ego is further strengthened because of two reasons;
a) It is an inherited ego i.e. they have to keep the feud alive because their forefathers had been doing so
b) It is families' collective ego and not individual one, do it is too hard to stop it.
Both families want to preserve their self (identity) by keeping the feud alive.