@sgtwiltan:
The rivalry between the Marines and Army are simply because of all the services, we are most alike. Friendly but a rivalry none the less.
Ultimately, the proof that an interservice rivalry is friendly and constructive can be seen when, in combat, the two rival services both shoot at the same enemy rather than each other. (At least as far as the real bullets are concerned. Non-lethal taunts are exempted from this principle.) A cornerstone principle of free-market economics is that competition between businesses is good for the consumer, so by analogy a bit of healthy interservice rivalry is probably beneficial in a similar way to the nation that these services defend.
It’s also my understanding that the US Army and the USMC have abilities that complement each other, with an overlap zone of common territory: the USMC is better suited to roles in which a small-to-medium sized rapid-reaction force is needed at short notice in odd corners of the world, while the US Army (which has more stuff but is slower to deploy) is on the whole better suited to conducting sustained-duration and/or large-scale operations. At least that’s the impression I have, from my limited perspective as a civilian.