For example, Schumann's Symphony No. 4 in d minor has the 4th movement end in D major. There are many many other examples that I have seen of this pattern; some that end on the major version of the same key, whereas others end on a different major key.
Is the reason purely to end on a "happy note"?
The only symphony that I can think of that declares it as minor and ends on the same minor key is Brahms's Symphony No. 4 in e minor.