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happiness may be unsuccessful is hedonic adaptation. Individuals are good at adjusting to their circumstances, so joy and irritation that they might experience becomes part of the background after a period of time, losing its impact on them. Gilbert et al (1998) have referred to a ‘psychological immune system’ that protects people from suffering too long or maladaptively from negative events. However, this immune system may also prevent us from experiencing lasting enhancements in subjective well-being. However, major events and psychotherapy can have sustainable effects. Intentional activities can also influence happiness (Sheldon & Lyubomirsky, 2007). 

(Killingsworth & Gilbert, 2010) - experience-sampling study. Sent out signals to people's smartphones at random intervals and asked them what they were doing and to rate their happiness in that moment. The activities where people were the happiest were making love, exercising and having a conversation. The activities where people were the least happy were resting/sleeping, working and on the computer at home.