In Look Back in Anger, the reason Jimmy Porter gives for his own anger is that he had to watch over his father's protracted death process from wounds gained in the Spanish Civil War. This suggests that there are twin reasons for Jimmy's anger. The first and deepest is psychological. The other is social.
Jimmy was only ten years old at the time of his father's death. This means that if he watched over his father for one year, his vigil as comforter began when he was nine and ended when he was ten. While he lay slowly dying, Jimmy's father talked and endlessly talked "pouring out all that was left of his life to one bewildered little boy." Watching at a tender age over your father's death is traumatizing in its own right but when that watching means being exposed to desperate discourse of a man trying to reconcile himself with the rationality of irrational ironic events, the trauma is compounded.
His father was a man of high ideals. England had joined with the U.S. and France in signing a Non-Intervention Pact that left the freedom fighters, fighting against Fascist Franco, without aid or reinforcements. Many individuals with noble ideals defied their countries and went to Spain to fight against Franco as mercenaries (Jean Brody in The Prime of Miss Jean Brodie advises one of her girl students to go fight in Spain).
This is what Jimmy's father had done. And all that had been gained for him and his family by it was a slow agonized death in front of his young son. We can only guess whether the father's ideas were still in favor of blind idealism or whether his impending death made him angry and disillusioned--or perhaps he talked trying to find his way out of a fog combining both. We do know that it all made Jimmy very confused, very alienated and very angry. We also know that other young men in England had similar feelings to what Jimmy had.
Jimmy's anger then is first personal and psychological with its foundation in youthful trauma of one of the worst kinds. Jimmy's anger is also social because his father's actions, leading to his death, were the direct result of social factors. Furthermore, Jimmy was inculcated one way or another by social precepts because of his father's experience and as the consequence of his vigil.