What if I tell you there's no such thing as a disappointment? Yeah, I know, sounds insane, but just give it the benefit of the doubt at least, let's see where we go from here, shall we?
Humans do not live in isolation, such is the design of our world. We are social beings, we have to interact with other creatures of our kind and otherwise. This is as a result of a system of mutual dependency we have on each other. The early men were self-sufficient in everything. From their food to medicine to clothes to shelter, they knew how to acquire and make these things themselves. But in today's world, we have been conditioned to be able to meet only a few of our essential needs, hence the general mutual dependency. Out of this general mutual dependency is where disappointment stems from. What do I mean??
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You see, growing up, I was an introvert. I didn't interact with other people much, including my immediate family members. I remember a period of three years in which the only words that were exchanged with my parents were ones of greeting. It was that bad. Now, in all these times of my hardened introvertedness, I also had a very limited desire. In my unwillingness to interact, I could not get most of the things I wanted, so I learnt to weather the storm of my urges and desires. In not interacting and not needing people as such, I learnt to expect little from people. The attendant effect from all this is that very scarcely have I been disappointed in my life by people or circumstances.
What am I getting at here? Much like every other human emotion, disappointment is a human social construct stemming from the expectations we have of others, which are usually of very little consequence. Naturally, man has no right to expect anything from their fellow humans, but the social design of mutual dependency is what has brought about that.
We shouldn't expect so much from other people. Because they aren't you, hence they are likely to fail you somehow. I have a standing principle "expect the worst, hope for the best". In expecting the worst, nothing can surprise nor disappoint you.
That expectation is a deception in itself. It draws from your thinking you can't be independent of social connections and interactions. Anyways, this is not to say one shouldn't have social connections and interactions, but with adequate understanding.