When you make plans you expect them to work out. And the key word here is expect. Which more often than not means disappointment.
Because plans usually involve factors other than just yourself. They involve places, inanimate objects maybe but never just yourself in isolation. So when you plan to read, the book could be horrible. When you plan to listen to music, your earphones could stop working. When you plan to use the internet, it would refuse to work. When you plan to go someplace, it could start raining or you could not have transport. Etc etc etc.
And then there are those that involve other people. These are plans which are most likely to, and 99% of the time always do, disappoint. These plans would require a mind other than your own, which means that you can't control it. Thus making it, you guessed it, fail. Because when you make these plans, you expect the other person/s to also think the same way. You expect her/him/them to do a certain thing or be at a certain place or stick to a certain time. Which further compounds the likelihood of said plans failing.
Solution to problem - Don't make plans and don't depend on others thus avoiding disappointment.
Reality - Learn to deal with the disappointment.