Expectation = a belief that someone will or should achieve something.

Expectations in a relationship are a sure way to ruin that relationship.
Whether the relationship is between friends, child - parent, spouses,
coach - student, siblings, coworkers, lovers, makes no difference; to expect someone
to do something is judgmental, unloving, disrespectful, and controlling.
Even when the expectations are held in one's mind without being expressed
verbally, they still affect the relationship, because subconsciously the
individual(s) involved can tell; their subconscious knowledge of the expectations
creates negative emotions, such as resentment, grief, anger, gall, fear, etc.
These emotions are bound to come to the surface in one way or another;
then the individual who was/is the target of the expectation(s) turns against
the expecter.   As well, because the act of expecting is unloving, it creates
an incongruence (disharmony) in the expecter's energy field and thus contributes
to the expecter's poor health and spiritual stuckness, and this can even affect
the experter's connection with Divine Loves (Gods), the source of life.

There is a difference between expecting and believing/trusting that someone
will do the agreed-upon work/activity.

At work the coworkers need to be able to trust that you will do your job
properly and honestly, because it affects their job.
At home the parents need to be able to trust that their child will cut the grass
as he/she agreed to do (with his/her free will).
You need to be able to trust the taxi driver that he will drive you to your
destination safely.
This trust is akin to relaxing into the situation and letting the individual (the doer)
handle it.   If the individual does a poor job there will be consequences for that,
especially when it's intentional (including karmic consequences).   However, you're
not projecting your mental stuff onto the individual; you're not trying to control him/her.

Also, there is a difference between expecting and encouraging.   A coach can congruently
encourage an athlete or a musician to improve his/her performance without expecting
a particular outcome.

Letting go of expectations is a must if you intend to live healthy, joyous life, and also
if you intend to progress spiritually.   If you feel an expectation forming in your mind,
it is beneficial to drop it, to ignore it.   No need to fight it and resist it.   Just dropping it
as if dropping a stone you were holding in your hand.   Also praying for Divine help
is important; for example: "I intend to be free of all expectations, and I thank Divine
Love for helping I with that."
At the root of most (if not all) disharmonious stuff, such as expectations, are toxicity,
energy/nervous system fragmentation, and lack of proper nutrition.
The page "Good Health Trinity" has more info on how to help with healing all that.
Reading of positive books, such as by Osho and Dr, David R. Hawkins, is of great help,
as well.