When you feel disappointed mostly it is because you expected a different outcome. Normally that outcome would suit what you want. When you are stuck relying on people mostly you have to accept the disappointment that comes with it. People normally end up doing the opposite of what you wanted.

I don’t think anyone is immune to disappointment because we are social animals. We do need people for a lot of things. Sometimes self reliance can look like giving up. In a way it is giving up disappointment. At least if things go wrong you can only blame yourself. Mostly with disappointment there is no right or wrong way of dealing with it.