Why change is inevitable in a project?

I would say that change in any project may not be inevitable but it is most likely to take place. The reason is that every project starts with a conceptual framework, which is never perfect. Humans tend to think different in different situations and most of the time towards betterment, that is what brings changes in projects.

'Change is the only thing which doesn't change '. Not sure you have a house of your own? Either ways, no matter how much ever houses you visit, review various architecture as part of planning and once you start constructing it, you will have n number of thoughts coming up due to many reasons. This is exactly what happens in any project as there is always a learning process throughout the project  and stakeholders get influenced by various aspects in the project and priorities and complexities may change and unknown unknown Risks will occur, Some of the least probable Risks become issues...etc etc.

-Abdul Rahiman

PMP,RMP,SP,ITILv4, CSM.

3 things which impact the outcome - (Triple Constraint)

Scope - Like human wants ever increasing.

Budget - No one wants to increase.

Timeline - We want everything and that too NOW!

However, there are a number of factors which play their tiny role in the outcome.

When we humans design something, we only plan for the seen and try to have a buffer for the unseen. However its not always that our buffer is sufficient to manage everything that we had not planned for. Hence CHANGE. 

- Fahad

Let's take an example from software development.  At the beginning of the project you gather needs from the customer.  As you start to develop the project and show screens to the customer, he starts to get new ideas and see things in a new light.  So as the Agile folks say, change is normal!

Because it is incredibly impossible to plan100%. You will always face issues which are not belongs to you but humain resources as well. Project is changing from time to time, from Budget, to new updated design for example.