Higher levels of serotonin are correlated with increased happiness and decreased anxiety. 
Dopamine also contributes to good mood, and in addition causes an increase in 
activity and willpower. 

Both serotonin and dopamine show decreased levels in depression.
However, it is important not just to look at general levels of the neurotransmitter, 
but to also consider which parts of the brain show decreased neurotransmitter levels. 
Depression correlates with decreased serotonin and dopamine levels in the 
amygdala, thalamus, and hyppocampus.