View update problem - Relational Model

In the words of the CEDANET notes:

The view update problem refers to the difficulty/impossibility of providing alternative updatable views on a database.

It's sometimes assumes views are associated with individual relvars.

It's more useful (and more general) to define a view as meaning a derived dbvar. Therefore a view means a set of named relvars. (just like the set of base relvars which are defined in the database schema).

It is assumed logical independence implies a view looks and behaves no differently than if the relvars in the view were declared as base relvars in a schema.

One of the purposes of normalisation is to avoid data redundancy and simpify updating the database. For that reason it is expected that an updatable view is normalised.

Why are views so difficult to make updatable? I don't yet have a good intuition for this.