Tragedy, Fate, and The House Of Atreus
The fate of the tragically doomed House of Atreus is both the source of many Greek myths, legends and plays and impossibly complex. We don't need to understand all the ins and outs of their history (and for this class you don't need to memorize what follows), but here's a simplified version, as relevant to Aeschylus' Oresteia:
First, note that Tantalus (Menelaus and Agamemnon's great grandfather) is the son of Zeus and the mortal nymph Plouto, so the origin of tragedy is, as in the Iliad and Odyssey, divine.
Atreus (Menelaus and Agamemnon's father) and Thyestes are brothers.
Thyestes sleeps with his brother, Atreus’, wife, Aerope. (She is the grand-daughter of King Minos, of Crete.)
In revenge, Atreus kills Thyestes' sons (so, he kills his own nephews) and has their bodies (everything but their hands and feet) cooked up in a tasty stew, which he serves to Thyestes -- "hmm...tastes like chicken, bro! What's in this scrummy stew?" "Psych! Your boys! Har har!" Then Atreus taunts Thyestes with the hands and feet.
Thyestes responds by seeking aid from the oracle, who advises him to have a son by his daughter, Pelopia (yes, his own daughter), and this son will grow up to someday kill Atreus. This son is Aegisthus.
However, ashamed of her incest, Pelopia abandons her infant son, Aegisthus . But Aegisthus is found by a shepherd and given to Atreus, who unknowingly raises him as his own son.
When Aegisthus reaches adulthood, Thyestes reveals to Aegisthus that he, Thyestes, is both father and grandfather to the boy (hey, like in Kentucky!). To revenge his brothers' deaths, Aegisthus kills Atreus (his uncle and adopted father), as was prophesized by the oracle.
However, Atreus also had two sons, Agamemnon and Menelaus, the two brothers who figure so centrally in The Iliad and The Odyssey.
As you know, Agamemnon married Clytemnestra, and Menelaus married her half sister, Helen (known later as Helen of Troy). Both share Leda as a mother, but Helen's father is Zeus (see Leda and the Swan), and Clytemnestra's the king Tyndareus. Of course Helen runs off with Paris of Troy during a visit; this love triangle was created by Zeus' jealous wife, the goddess Hera, to punish Zeus and Leda for their adultery. Menelaus then called on the chieftains to help him take back Helen...and thus begins the Trojan war as told in The Iliad and Odyssey.
What All This Means:
For the Ancient Greeks (as well as Ancient Hebrews: see all humans punished for Adam and Eve's transgressions), the sins of the father are visited on the sons...or daughters, or grandchildren: Fate, or the gods, will justly punish the children of someone who has broken a law or ritual; this punishment, however, will occur through human actions; that is, the gods don’t directly punish sins but rather this theory explains why bad things happen – when something bad happens to a person it is because of some past family transgression.
Thus, Agamemnon's fate is tied both to his own actions (killing Iphigenia) and his father's murder of Thyestes' children. This holds true for most of the players in the Oresteia.
So, traditionally, for the Ancient Greeks, everything happens due to some cause, but that original cause may have occurred generations before our birth.
Our fates are wrapped up in a complex web that is mysterious and divine, and the fate of humans may be because of the folly or weakness of a god -- so in this case the House of Atreus is punished for Zeus' lust and adultery, by Zeus' jealous wife, Hera.
Because of all this, our fates are often inherently tragic and, well, fated, which is to say: beyond our control.
Classical Greeks like Aeschylus in his Oresteia are questioning this traditional belief.
Also see Tragedy, Fate and Hamartia