The fly and the robin

Here is a little question for you to ponder while enjoying your “Friday night is cocktail night”
 
A few days ago, I saw a magpie swoop down and catch a small bird, probably a recently fledged robin, and carry it off to feed its own chicks.
 
I felt terrible and have been swearing at the magpies in the garden ever since.
 
Today, while trying to get a decent photo of this fly with my 60 mm f2.8 macro lens, a robin swooped down and caught the fly!.
 
I was able to grab just one shot of the robin and off it flew.
 
Apologies for the poor quality of both photos, but the question is
“Why did I not feel bad about the demise of the fly?”
 
I had a small exchange of ideas earlier in the week with 13south who asked why we VALUE small, cute birds more than large ones, or in my case, a fly.
 
No answer required – just think about it