One important characteristic of the 2nd movement of symphonies in the 18th and 19th centuries is that they are almost always in a different but related key center. The first, third, and fourth movements remain in the same key center.
Do you think that this is just a matter of convention or tradition or is there possibly another reason (or other reasons) for this change of key center for the second movement?
Another aspect of the 2nd movement is that it is usually slow and melodic (lyrical). How does this fit with the change of the key center? Do you think there is a connection between both of these changes from the first movement?
As a part of this discussion, present your own ideas about the importance and/or limitations of relying on tradition and convention in musical compositions.