The past no longer exists, and if we try to live in it via memory we will become depressed.

The future does not yet exist, and if we try to live in it via imagination we will become anxious.

The present exists, and if we live in it we will be at peace.

Yet, we can exercise memory and imagination now in the present. We have developed these faculties to aid us in dealing with the present moment and in understanding the present moment more thoroughly. So to form an expectation may be a cause of anxiety or it may be an aid to living in the present moment more successfully.

I believe the key differences are our intention in forming an expectation (ie whether it is to equip us to take some action should a contingency arise) and our willingness to let go of a certain expectation if things turn out differently.