What is the result of an excess of dopamine? I'm pretty familiar with the results of having not enough dopamine, though the reasons are not so clear to me.

To get a better insight on the topic I'd like to know something about having too much dopamine:


What behaviour is result of having too much dopamine?
Why does it happen?


I am aiming to get insight in disease states associated with excess dopamine, not drug-induced states.
Too much dopamine release in the striatum may lead to psychosis, and especially to the positive symptoms associated with this disease (e.g., delusions and hallucinations, as opposed to the negative symptoms such as a flattened affect) (Laruelle et al., 1999). 

The etiology of psychosis is complex. Many direct and indirect environmental factors are implicated, as well as genetic factors. Environmental factors include obstetric complications and frequent cannabis or amphetamine use. The environmental factors are all implicated stronger in psychosis than genetic causes. Genetic predisposition and environmental factors may also interact in complex ways in the development of psychosis (Howes &amp; Kapur, 2009). 

References
- Howes &amp; Kapur, Schizophrenia Bulletin (2009); 35(3): 549–62
- Laruelle et al., Biol Psychiatry (1999); 46:56–72