Answer:
This isn't an easy question to answer, and my attempt is in now way meant to be conclusive. Anger is essentially the basic emotion on which Jimmy Porter thrives, the one emotion that enables him to cope with the disappointment that lurks in every aspect of his existence. Prime target for his angry diatribes are of course his wife Alison, and to a lesser, more ironic degree, his flatmate Cliff. When Helena arrives, she also becomes victim of some of Jimmy's angry fallings-out.
Jimmy's anger is partially fed by class hatred, exemplified by his railings against Alison, her family and her friends (Helena.) Then there's the existential anger, about not getting anywhere: the Porters are stuck in a dull Midlands town, in uncomfortable and drabby lodgings, and university-educated Jimmy has to make a living running a sweet stall. This anger relates to his feeling of life not turning out as he expected.
Furthermore, his anger seems to be motivated by his feeling of being born in the wrong era. Even though Jimmy despises Alisons father for his colonial Edwardian past, there seems to be a fair deal of nostalgia as well, a time when a man could still live (and more crucially: die) for what Jimmy calls good, brave causes.
"I learnt at an early age what it was to be angry and helpless, " Jimmy rails. This relates his anger to a basic trauma he endured in childhood: in one of his more vulnerable moments, Jimmy reveals that as a child he was a close witness to the death of his father. The hypocrisy of his family regarding this event, taught him a great deal about anger.
I hope this answer has been useful to you. "Look back in anger" is a particular favorite of mine. If you have any more questions, contact me through twitter: @michielleen.