Not as pleasant as beet soup.
(1) In ethology, "zugunruhe" is anxious behavior expressed by migratory animals (especially birds) that are prevented from migrating when they normally would. When these animals are enclosed they exhibit this behavior in seasons when they would normally migrate. It's that sudden desire that everybody gets from time to time — to be somewhere else.
Haven't gotten it yet? You will. If you're a bird, then somewhere else is another hemisphere. Maybe you're a wildebeest. Same story, but you don't have to flap anything or go so far, but you're itchy to move all the same.
If you're a human, and you have any sense at all, then somewhere else is on the trail. So go already. If you already are on the trail, then your urge resolves to seeing what's over that tantalizing horizon up ahead. (Don't worry about this, it's a totally healthy impulse and gives you a reason to get out of bag.)
(2) Cubicle fever. If you're unlucky enough to have a job, and they've put you in a carpeted box, and it's all frayed because you've been gnawing on it like crazy, out of terminal frustration...well, it's time for a change. If you don't have any vacation time left, and can't lie your way into an educational sabbatical, and are too poor to up and quit, then let your insanity blossom and shoot for a medical disability. Then go backpacking. With abandon. (You'll be welcome to hike with the rest of us.)
(3) Butt itch, either physical or spiritual. First try washing and applying soothing lotions, and then up your mileage until you reach the onset of permanent euphoria. But keep washing too, especially before you come over for a visit.
(4) SABR: Seasonal Affective Backpacking Rapture.