Because it literally includes how the mind thinks (definition: the fact of being who or what
a person or thing is.)
And it’s hard to change. But unless it is an outrageous one like a
serial killer, being with authentic individuality shapes all the
experiences, decisions, and perceptions. The perception of life lies
upon the identity itself.
And people thus rarely do things not linked to their identity, like a
weak elder’s identity is not about lifting weights. But that’s fine.
When what people do is not linked to what they are willing enough to do,
it’s one of the few times where even great minds feel immense pain, and
it’s the undesirable type of pain too (too intense that it can become
hatred).
If you pre-dream an identity after doing a task that is not guaranteed to succeed, you MAY feel worse after failing because the brain hates losing what it already planned for (more emotional significance in dreams than the actual loss itself).