Here's a short blog post that attempts to explain deployment nostalgia. However, I'm not sure it's something that can really be put into words. One has to experience it for yourself before you truly understand.
To counter the above, a top ten list of why deployment nostalgia is bullshit. The reality is that deployments are horrible. I have experienced and agree with most of what's on this list. Especially Ali Al Salem Airbase, it really is purgatory on Earth. There's just no other way of explaining it.