One of the many Buddhist teachings is to not overindulge in anything. This includes feelings. Meaning, when something happy happens you wouldn’t get too happy, and when something sad happens you would get to sad. This way nothing in life would be tragically sad, but at the same time you would never be overwhelmed with joy. This would help you keep at peace with yourself by not letting overreact to any situation.
This same thought process can be applied to the idea that if you don’t have any expectations, then you will not have any disappointments. How can you be disappointed if you don’t expect anything in the first place?
Exactly. No expectations? No disappointments.