I think some scientific insights on this can come from a couple of sources:
This is a branch of psychological therapy that interprets the human experience as follows.
There is an event, then the evaluation of that event, and then the emotion. Disappointment would be the end result, the emotion. Regular therapy might focus on avoiding the event, the trigger. In cognitive therapy one would try to change the evaluation of the event, and as a result, change the triggered emotion.
To put it simply by example, if you were once bitten by a dog, you evaluate seeing a dog as a threat, and therefore experience fear when you see one. By learning to evaluate dogs as friendly and nice, the fear goes away.
So, this seems to imply that there might be things you can concentrate on that make it more likely for negative emotions to go away. But you would have to look into the details of cognitive therapy to figure out how exactly this works.
Diener and Seligman have done quite some research on learned optimism. This includes the question of whether it's possible to "teach" a pessimist to become an optimist. In one of their studies (Diener & Seligman, 2002), they have identified the 10% of happiest people in a group of 222 undergraduates, and compared them to the unhappiest students. Both experienced the same amount of negative emotions, but the happy people got over it much faster because they had different cognitive interpretations of the events.
There are also some studies, not sure by who, that have shown that applying simple techniques can turn you into an "optimist" . A few studies (e.g., Johnstone, van Reekum, Urry, Kalin, & Davidson, 2007; Ochsner, Bunge, Gross, & Gabrieli, 2002; McRae et al., 2012; Hecht, 2013) were able to see brain activity changes and brain-density changes in the prefrontal cortex. In "optimists" the left side is more active. In "pessimists" the right side is more active. So this might support the brain and "chemicals" claims. But you'd have to Google the exact reference.
Broaden and build
Barbara Fredrickson has some interesting research (e.g., Fredrickson, B.L., 2001) on how negative emotional states limit your range of reactions to "choose" from in response to a situation. And how positive emotions increase this range. She calls it the broaden and build theory. The most limited range on this spectrum is the fight or flight reaction in times of great stress and danger.
She does describe some "spiral" effects, where the negative emotional states (e.g. disappointment) can lead to cumulatively more restricted mindsets.
Also, maybe this is worth mentioning. There is a lot of research supporting the claim that meditation increases emotional stability. Mindfulness meditation specifically has been associated with the same changes in the brain as described earlier, associated with optimists. The "technique" taught here is to observe all events and emotions as if looking at them in third-person perspective, letting emotions rise and fall as they come without clinging to them.
This would indicate that it's not the "concentrating on disappointment" that is the problem, but it is the "type" of concentrating.
I think on most of the above topics you should be able to find references quickly on Google. I don't have time to look up the actual papers...
Diener, E., Seligman M.E., (2002). Very happy people. Psychological Science, 13(1):81-4. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/11894851
Fredrickson, B. L. (2001). The Role of Positive Emotions in Positive Psychology: The Broadenand-Build Theory of Positive Emotions. American Psychologist, 56, 218-226. Retrieved from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3122271/
Hecht, D. (2013). The neural basis of optimism and pessimism. Experimental Neurobiology, 22(3), 173–199. Retrieved from http://synapse.koreamed.org/search.php?where=aview&id=10.5607/en.2013.22.3.173&code=0142EN&vmode=FULL.
Johnstone, T., van Reekum, C. M., Urry, H. L., Kalin, N. H., & Davidson, R. J. (2007). Failure to regulate: Counterproductive recruitment of top-down prefrontal-subcortical circuitry in major depression. The Journal of Neuroscience, 27(33), 8877–8884. Retrieved from http://www.jneurosci.org/content/27/33/8877.full.
McRae, K., Gross, J. J., Weber, J., Robertson, E. R., Sokol–Hessner, P., Ray, R. D., Gabrieli, J. D., & Ochsner, K. N. (2012). The development of emotion regulation: an fMRI study of cognitive reappraisal in children, adolescents and young adults. Social Cognitive and Affective Neuroscience, 7(1), 11–22. Retrieved from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3252634/.
Ochsner, K. N., Bunge, S. A., Gross, J. J., & Gabrieli, J. D. (2002). Rethinking feelings: An fMRI study of the cognitive regulation of emotion. Journal of Cognitive Neuroscience, 14(8), 1215–1229. Retrieved from http://brainimaging.waisman.wisc.edu/~perlman/0903-EmoPaper/CogRegEmoOschner.2002.pdf.