Trust.

What does it mean to trust somebody?
You believe what they say is the truth.
You believe that when they lie, they do it for your own good.
You believe they don't judge you.
You believe they will take care of you.
You believe you can depend on them for anything.
You believe they will never judge you.
You believe they will never hurt you intentionally.
You believe they will always make it right when they hurt you unintentionally.
You believe they always have a solution to your problem.
You believe they will always be sympathetic and caring.
You believe they will always be the same.

Why is it hard to trust people?
Because no one can be all of that at the same time.