Disappointment is a psychological event of course, but in this page, it will be used to describe a biological phenomenon. Every living organism expands and contracts. There are things that induce expansion, usually pleasureable, and things that induce contraction, usually painful. Disappointment is the proven biological fact that after something has caused contraction the things that normally lead expansion are much less effective. This is captured in the folk expression, "once bitten, twice shy."
Wilhelm Reich used the example (not just a metaphor) of the amoeba. An amoeba in solution will send out pseudopodia. If poked however it will contract suddenly. After this, it will send out pseudopodia but at less distance, it seems almost cautious. Amoebas are one-celled with no brain or nervous system, so this reaction must be cellular.