The problems between thumb and index

[Thea7]

Hello everybody,
I am really confused about the "problems", when using thumb or index.
For example in lesson 6:
There are several string changes, where index is on the lower string and the thumb on the next string.
Why is it suddenly " a problem" when it comes, for example, to the part D string to G string....?

I hope someone understand, what I am talking about :)
its really difficult to write about these things.
Thank you Emil for doing this!