I think it's worse for me after big trips - such as a thru hike. When it's over, you are often "ready" for it to be over - in the sense that it's a long, long journey and frankly, you are often a bit tired and ragged toward the end, especially if the weather is turning. But it's also depressing in the sense it's something you've planned and worked for a long time - and when it's over the goal of finishing seems something of a letdown, actually. I was excited to reach the northern terminus, but at the same time, rather sad a fun adventure was over.
But get back to "civilization" and go to work where you need to concentrate all day; that's far more tiresome than hiking. Backpacking is pretty simple - walking punctuated by the need for water/food, the occasional call of nature and sleep.
"Normal" life is far more complicated and aggravating - and depressing. While I was glad to be off trail for a day or two (the opportunity to spend time with my wife again was fantastic), I felt out of sync for a long time. I agree that planning another trip or going bigger are fantastic ideas - as is making a project of your photos.
Hiking is often that way - when I am out I often miss things back home but when I am at home, I miss the trail.