Your Assumptions distorts you Perceptions/Reality, which affects your Expectation and therefore your Attachments, which leads to disappointment
When you stop making Assumptions about outcomes and people, your Perception or your Reality Changes and becomes clearer, you stop trying to control everything and having unrealistic Expectations and let go of Attachment.
When life or relationships are not meeting your expectations,let go of how you think things should be. It a matter of not having assumptions or attachments to a fixed outcome or perception. Just let go!