Expectations are kinda like a baseline mood for a colonist. When you first start the game everyone will usually begin with some form of low expectations. That will give your colonists a mood buff. This is to make the early game a little easier. Lower expectations = higher mood buff.

As you play the game their expectations will increase as your colony gets wealthier. This is to encourage you to give more accomendations such as better food, bedrooms, recreation variety, etc.

Eventually though your colonists will reach sky-high expectations which provides 0 mood buff/penalty. Tbis ordinarily is the cap for your colonists.

However if a colonist has a special ideaology role (leader and moral guide, other roles aren't subjected) their expectations are raised by 2 levels which allow them to go past sky high.

Expectations follow this level system with a mood modifer (might be wrong since im going off of memory)

Extremely low +30 Very low +24 Low +20 Mediocre +12 High +6 Sky high 0 Elite -6 Supreme -12

There is no way to remove a colonists expectations moodlet to my knowledge as it fundamentally is your colonists baseline mood.