Why does unity reimport the assets every time I open a project? [SOLVED]

The problem is that when a project reimports its assets, all the scene of the game is empty, that is to say,everything that I have done in the scene it is deleted. And I need to start again from the beginning. How can I deal with this?
1 - Is there any way to save my scene when the project needs to reimport the assets?
2 - And why does it constantly need to reimport the assets???
Thanks for all the answers!

Alright :)) Well, I downloaded the Standard Assets Pack and that was what was causing all that troubleā€¦ When I deleted it (mainly the scripts), the reimport disappeared. So since then I only use objects and prefabs created by me and it works greatly.
I hope it will help")