What is the result of an excess of dopamine?
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