Why is a difficult question to answer. I think everyone wants to get away from what they know and to see and explore different places. I remember when I was a kid I used to love looking at atlases and wanted to travel to all of those oddly-named and far-flung places. Now I'm nearing 25 and I've never been outside of Western Europe. There's nothing wrong with that, except that it makes me feel like I'm wasting something.
There's also that peculiar sense of joy or tranquility even when on a fairly short journey. Just being out of the main loop, away from all of the problems you faced at home is very refreshing, even if you face others, and even if the old ones are still there on your return. The crappy times still make good stories - it doesn't matter, it's something different.
Where? Good question. Mongolia - hang out on the steppes, y'know. I'd love to go to North America and walk around in some of those forested, mountainous places you see in photos and that we just don't have in Ireland, where our mountains are really hills and nowhere is still wild. I really want to do another giant walk, across America or Europe. Botswana has been in my head for years simply because it's one of the only places I can't remember every hearing anything about, and I want to see what's going on. Scottish highlands. Siberia. Brazil. The Faroes. Mexico. New Zealand. Perhaps part of the problem is that I don't have one particularly strong fixation.