Why our brain can't fool itself to constantly produce dopamine?
Short answer
Continuous activation of the reward system leads to habituation and hence a downregulation of the pleasure feelings. Further, a continuous euphoric state is a potentially harmful state, as behaviors essential to life are undermined. 

Background
I'm having some difficulty what you mean with a brain fooling itself. A brain cannot fool itself. More importantly for the question, however, is that in case of the reward system it definitely doesn't want to 'fool itself'. Below I outline why from two different angles.

The evolutionairy perspective
In its very essence, the reward system is one of the most ancient parts of the brain and the use of dopamine neurons to mediate reward-driven behavior is also seen in worms and flies, which evolved around 1 billion years ago (Mount Sinai Medical School). Indeed, it's evolutionary role is to stimulate behaviors essential to the survival of an individual such as eating, but it is also essential for the survival of the continuity of an entire species by stimulating the drive for sexual reproduction (Melis &amp; Argiolas, 1995). Reward should follow beneficial behaviors - continuous activation wouldn't aid anything other than a continuous euphoric state. An individual being euphoric wouldn't survive long as it wouldn't pursue anything any longer. That would quickly lead to a discontinuation of that trait as the individual would not be fit to reproduce (Darwin's survival of the fittest). 

The physiological perspective
Continuous activation of the dopamine pathways quickly results in habituation (increasing resistance to the drug's effects) and, in the case of illicit drugs, to dependence (craving/compulsory drug seeking), together referred to as addiction (compulsive drug use despite harmful consequences). The downregulation of the receptors for dopamine (mainly D2) is a physiological reason reward sensations should be transient and uncommon, otherwise the reward system is being tuned down.  

Reference
- Melis &amp; Argiolas, Neurosci Biobehav Rev (1995); 19(1): 19-38