Can we change our dopamine baseline levels?
Feelings of pleasure and reward are transient in nature. Similarly, the dopamine release in the reward centers (the limbic structures) is transient, namely in the order of seconds (Rebec et al., 1997). 

One way to chronically elevate dopamine levels is by administering certain drugs. A notable/notorious example is methamphetamine, which elevates mood and generates euphoria for 8-12 hours. However, after chronic use, people may report a lack of motivation and anhedonia when they stop taking the drug. Anhedonia is an absence of pleasure in response to acts that had previously been pleasurable. Anhedonia may be reported as long as 2 years after the last use of methamphetamine in addicts. Chronic use of the drug overstimulates the dopamine neurons in the limbic system (and elsewhere), and destroys the dopamine receptors, explaining the anhedonia. 

Hence, even with hard drugs like meth, the effects on dopamine levels are transient, just less transient as with physiological reward responses.

Reference
- Rebec et al., Brain Res (1997); 776(1–2): 61–7