What causes the hatred between POGs and Grunts?

Hatred is the result of ignorance (on both sides).

Combat Arms of all flavors, requires support. You can't take a hill without beans, bullets, and bandaids, or the people who do all the paperwork behind the scenes. Without Combat Arms, those fields would have no need to exist. It's a symbiotic relationship.

Generally at the lower ranks, this turns into a have vs. have not issue though, because training camps have a requirement to be located in (pardon the phrase) crapholes. Infantry needs to be near deserts, mountains, or swamps to train. Non-Infantry, such as the Air Wing or Logistics need to be near Logistically viable locations with "roads" and the ability to move equipment. This in turn means that they have better stuff, just as a matter of practicality. Why put good stuff in a swamp or a desert when you can fly it or truck it in to a perfectly good location.

So the standards of living just end up being "different," and by different I mean better. Oh so much better. And I say this after doing 4 years with an Infantry BN. This doesn't make things "bad." Camp Horno wasn't bad when I was there. It was just in the middle of nowhere, surrounded by mountains. If you were to compare that to mainside or even the wing, we probably probably had nicer barracks.. but their proximity to "civilization" and not having the same optempo & training schedule just makes it seem like a better life.

Grunts are basic folks who live in mud and sling bullets. They eat weird stuff and smell funny. By comparison, POG's are extremely intelligent. They live in the barracks, take showers (sometimes twice a day), dine on lobster tail, and smell like lilacs. Most grunts who lat move into a more... how should I say... "administrative" role, usually have an epiphany: POG's work is hard in its own right.

The Corps is somewhat unique: The over-riding philosophy is developed from OCS or Boot Camp: We're all Marines FIRST. Regardless of the MOS, our mission is unified: To locate, close with, and destroy the enemy. Some just have to do this from behind a monitor. Regardless of MOS, we all support the warfighters. Those who have re-classed just grab onto this concept earlier.

It's because their sole existence is to support the Infantry! I'll take it even further every infantry MOS like machine gunners and mortar men exist to support the rifleman. Inside the all volunteer military the guys who sign up for the shittest job (infantry) take a lot of pride in that. The POGs enjoying lording their "superior intellengence" over the "dumb grunts". I've haven't seen that many movies about POGs but their are a lot of movies about the infantry and their special operations cousins. Haters gonna hate!