Because of how he trained her Regina had to always hold on to that anger.
Her ass got excited and happy for two seconds for doing something right then nope it went away because happiness and joy is not the way she was taught to tap into her power.
But with the annoyance she makes it flair up again. She had to constantly stay in touch with the worse of herself in order to be protected..because without magic she was just Regina with a dead fiancee, an abusive mom , a scared ass dad, a perv old ass husband , and a kid sister /step daughter. No allies, no power, magically trapped in a kingdom..