Eating sweets does improve mood. There are several known mechanisms. e.g., carbs facilitate amino acid transport across the blood-brain barrier [1], and wheat stimulates endogenous opiate receptors [2]. But, if you want to influence brain chemistry therapeutically, eating sweets is probably not the best approach. Good luck!

[1] http://www.ncbi.nlm.nih.gov/pubmed/3527063 [2] http://www.heartscanblog.org/2011/01/heroine-oxycontin-and-w...