Final answer:
The personality trait that is characterized by always looking on the bright side and being very positive is emotional stability, part of the Big Five personality traits.
Step-by-step explanation:
If your best friend always looks on the bright side and is very positive, the personality trait they are demonstrating is most likely emotional stability. This trait is one of the Big Five personality traits and is characterized by individuals being stable and calm, and they tend to have a positive outlook on life. In contrast, the trait neuroticism, which is at the other end of this spectrum, would involve a tendency towards negative emotions and a pessimistic view of the world. While traits such as extroversion and agreeableness also involve positive interactions with others and a friendly nature, the key aspect of consistently seeing the bright side of things points to emotional stability. It is important to remember that personality traits are part of a complex system and can interact in different ways; thus, a person who is emotionally stable might also show high levels of other positive traits such as agreeableness or conscientiousness. However, based on the description provided, emotional stability is the trait that primarily captures the tendency to remain positive and optimistic.