Why do Expectations lead to Disappointments?

'Expectations lead to disappointments',

The proverb states that expecting always leads to resentments.

However, why does this happen remains the question,

The answer to which isn't caging your expectations in motion.

Expectations provide hope,

Which help us recover from life's evil slope.

Expectations are actually assumptions,

Which make us form resolutions.

These speculations may end up faulty,

Which is the reason of our life being salty.

One mustn't predict their life through mere assumptions,

Since these differ from reality, being imaginations.

Expect something to happen but don't expect things to happen your way,

This is my ideology, this is my theory, I say.