Final answer:
David's tendency to feel often depressed is described by the Big Five personality trait of Neuroticism, which is associated with negative emotions and emotional instability.
Step-by-step explanation:
The Big Five trait that describes David's tendency to often feel depressed and easy to feel down and blue is Neuroticism. This trait includes facets such as anxiety, emotional instability, and moodiness. Individuals high in neuroticism are more likely to experience negative emotions like sadness and anger and can be characterized as impulsive and hostile. This trait is on a spectrum with stable and calm at the lower end and anxious and unhappy at the high end. Neuroticism has been identified as a risk factor for chronic health problems and mortality. In David's case, his feelings of depression would indicate higher levels of Neuroticism within the Big Five personality factors.