Act the way you want to feel and soon you will feel the way you act.
This is common in self-confidence building therapies or courses where you are asked to act the part of or like a confident person. You soon develop that skill or behavior which results in the person having more confidence. There is increasing evidence that acting enraged, obsessed, malevolent, or depressed maybe bad for you. Actor Leonard DiCaprio developed obsessive-compulsive disorder while playing Howard Hughes in the block buster The Aviator. This often happens to actors who get “caught up” in the role they play or keep many of the same character traits of the role they played. Heath Ledger who played the Joker in the Batman movie died and was allegedly clinically depressed. If you act having an enraged or angry conversation with someone, you will usually find that your emotions do get engaged even though you are simply acting the part.
When you are acting happy, you are thinking happy thoughts that go with the acting role you are playing, so it’s like conditioning your mind to be happy.