Why is "gradle build" every time I open Android Studio

Question

I am a little new to Android development. I noticed that every time I open Android Studio (which then tries to reopen my most recent project), I see that "gradle build" starts.

In my opinion, if gradle is like "make", it should be built only when something has changed, or I am forcing the project to rebuild.

So why does it work just by opening a project? The sluggishness of Android Studio and gradle are two big turns for me as a whole, as a new Android developer.

Thanks michael