How come Megadeth fans never hesitate to bash Metallica?

It seems that whenever I go on a metal forum (or any music forum that
matter) whenever someone mentions Metallica or Megadeth, the Megadeth
fans won't hesitate to bash Metallica without even being provoked...
Thoughts?

P.S. I like both Megadeth and Metallica, but I won't lie, I'm more
partial to Metallica.