For me:
Japanese. I have tried it. First time in 1987-'88 from a little 'Get By In Japanese' from the BBC. I did one more serious try about 10 years ago. Whilst I like traditional Japanese culture, I can't abide the popular culture, the demented cartoon rubbish and all the other stuff. I don't have a burning desire to live in Japan. So I find it hard to engage. Plus the fact that it's quite difficult! As much as it would be great to speak and read it, I am willing to accept that it's not going to happen in my lifetime.
Mandarin. This is one of those where you keep thinking 'hold on though... they say the grammar is not that hard, and no verb conjugations to indicate person, number, or gender! Granted, there's scary 'aspect markers', but I'll get over that...'. Then you start the course and you're like a one-legged man in an arse-kicking contest. I find this distressing because I like Chinese. Then there's the writing system...
Arabic. Many want to conquer this. I don't know how many succeed overall. I tried for a week and couldn't carry on. For simple westerners like me the writing system is an obstacle; the sound system too. I think that to learn Arabic I would have to give up every other language and probably some other things. Sadly I don't think I will ever learn Arabic and it's pity not to be able to read books that rarely if ever make it to the west in translation.
Turkish. I really wanted to learn Turkish. I had a good go. It has some attractive features from a Euro point-of-view, such as the alphabet, though this is misleading. There are lots of Turkish people I could talk to, but somehow I keep not learning it. I imagine this will persist.
You never want to say never, but I'd guess that I'll never learn these languages.
) and Japan contributes a huge amount to that. 
Not to mention the overall natural beauties of the nation I'd like to one day see.
