These are just some of the things that I love about Japan. This is by no means a complete list, but more like a summary of everything that comes to mind when I get asked that question.
1. Living here is very convenient. Many shops are 24/7, and government instances such as city halls are never crowded and provide good service. All of your mail is delivered on the door for no extra charge, even big parcels.
2. Housing is readily available and relatively cheap. Where I come from, people put themselves on waiting lists to even have a roof over their heads when they start studying. I have yet to hear about a Japanese university which did not provide at least temporary housing for its foreign students. When I moved out of my temporary housing, I just went to the real estate agent, picked out a room, got the contract in the mail a week later and just moved.
3. Infrastructure is very well maintained. Driving in Japan is, unless you live in a big city, very convenient. In addition to this, road-tax for small cars is pretty much non-existent (something like 2000 yen a year).
4. Public transport will take you anywhere, and is practically never late. It is also fairly reasonable if you buy a daysaver or a monthly pass.
5. Eating at restaurants is cheap and fun. If you want to go out to eat and drink (which is probably the most popular Japanese pastime...), you can just drop into any roadside Izakaya to enjoy great Japanese food. Where I come from, eating at a restaurant is so expensive that you would only do it on special occasions. Here, even students can afford buying a ready made lunch whenever they feel like it. Which brings me to...
6. The food. Japanese food is just as tasty as it is beautiful, so much so that many Japanese girls actually take pictures of their food before they eat it. Even much of the western food is better here, because the Japanese chefs take the good parts of western food and improve it. Believe it or not: In Japan, burgers from McDonalds actually look like the ones on the pictures!
7. The people. The overwhelming majority of people that I have met here are really friendly.
8. Onsen. Japanese hot springs are amazing. It is quite the culture shock to begin with, but once you learn to block out the fact that you are completely naked in a large bath-house with 20-30 others, you start to realize how great it really is.
9. Karaoke. In Japan, people don't go to night-clubs, but spend the night singing karaoke until the early hours.
10. Japanese people are really talented. It is common for a Japanese person to choose a hobby at a young age, and just stick to that their whole lives. Because of this, you find people all around who are just really good at what they do.
11. Public places are mostly spotlessly clean. If you take the first train in the morning, you can see the station staff cleaning every corner of the station manually. They even take time to wipe the walls! Also, the city rarely provides garbage cans in public areas, but most people still take their garbage home and recycle it.
12. Public toilets are clean, everywhere and hi-tech. Who would have thought that you can just walk into a random public-park toilet and sit down on an electrically heated toilet seat?
13. The summer. In the summer, Japan turns into a summer paradise, with long clean beaches, great temperatures and good conditions for swimming, surfing and other summer activities. Downside: 35 centigrade exam period...
14. Clean air, by Asian standards.
15. Student housing always has a bathtub.
16. The whole country is ridiculously safe. People leave their bikes unlocked when they go to the store, pick pocketing is almost not an issue, lost items are mostly returned. The areas here that are known to have a high crime-rate are just like anywhere in a European city. Also, girls can walk home alone drunk wearing a mini-skirt without any issues. That is not to say there isn't crime, though. I have friends who have gotten their bikes stolen, or their houses broken into. There is just a lot less of it.
17. Relatively few homeless people and addicts.
18. Government sponsored health care which is almost free for students.
19. Low taxes on everyday items such as groceries and gasoline. Electronics and household appliances are also cheap.
20. Smoking is banned in crowded areas, which have designated smoking areas. For example, there are only a few spots on campus where smoking is allowed. Strangely enough though, smoking indoors in restaurants and bars is totally fine.
21. Amazing customer service. Employees will go out of their way to help you even though they speak no English.
Those are just a few reasons that came to mind, I'm sure there are many more that I just didn't think of.
Lastly, let me add that I love my home country for just as many reasons, and that neither that nor Japan is by any means a perfect country!