What is the main theme of the poem and what does it have to do with the ups and downs of love?

In my opinion, the main theme of this poem is loss.  The
speaker has lost the love of his life and is going to be sad for the rest of his
life.

I think that there are not a lot of ups and downs in
this poem -- just one big up and one big down.  The huge up is when they fall in love
and she become his.  He says their love is so great the angels are jealous of
them.

But then she dies.  And there surely cannot be much
more of a down than that.  So it's not exactly ups and downs of love.  It's more like
the "up" of being in love but then a big "down" of her dying.