I guess it’s the same in most western countries as in Sweden: slurping when eating soup or noodles is considered to be bad table manners. Children gets chided in early ages not to do so. And if you do it among you’re friends, it’s usually ’cause you want to annoy them. ^_-
But lo and behold: in Japan it’s the absolute opposite! You can slurp as much as you want (well, ALMOST as much as you want, depending on situation). So don’t get confused if you’re Japanese friend are making loud noises next to you.
It’s actually considered to be good table manners and often you are expected to make slurping sounds when eating noodles like ramen, soba or udon. That’s a sign of that you are enjoying the meal and as louder you slurp, the bigger appreciation and enjoyment you express. I can tell by my own experience that I find it hard to do so, basically ’cause I’m raised not to. I actually ate a bowl of udon last night and I found it awkward to slurp even though I was alone. Ha ha. ^_-
Some people says jokingly that the best way to find the best ramen restaurants in Japan is to pop in your head and measure the loudness of slurping-sounds. However it’s true, I don’t know, but it sure sounds funny. ^^
In other words: slurping is a way to show that you are enjoying your food, and if you don’t slurp, it might actually be regarded as a bit offending. Many people also say that the food tastes better if you slurp. Does it?
Important to note though, is that TOO loud slurping sounds may be considered as rude among some Japanese people, so don’t overdo it.
There are SO many rules considering table manners in Japan. It’s truly not easy to manage them all as a westerner. ^_-