哎...I'm so tired of being asked this... Every single language is hard to learn, so Chinese is not unique to that. Take a Spanish class and see if you can roll your r's. It takes time, practice, and most of all motivation-because you can spend years learning a language, but if you don't have any motivation to learn it you won't do very well. "Difficult" is not synonymous with "impossible". Just because you feel Chinese is hard to learn, it doesn't mean its impossible to learn. Some people are very good at learning foreign languages,and some people aren't. I've been learning Chinese for about 6-7 years. I can read, speak and write it. Chinese people always think I'm Chinese when we talk on QQ until they see my pictures on my 空间....however I know people who learned Chinese for a while and can't say more than "你好“ and "汉堡包". What's the difference? Well, I'm very passionate about learning foreign languages and some people are not.
I don't think "我觉得中文很难“ or "为什么想学中文？你不觉得很难吗？” is something you should be telling Chinese learners because it will only discourage them. This may come as a surprise to you, but I hate being complimented on my Chinese. Mainly because of phrases like:"哇，你的中文很好！而且中文不好学。。“Emphasizing how hard you feel Chinese is to make me seem "很聪明" or "很厉害" doesn't make me happy, it actually makes me feel sad, as if Chinese people think I'm too stupid to learn Mandarin. If you want to compliment someone, just speak to them IN Chinese. That's a compliment in itself.
Maybe they are learning Chinese because they like your culture? Maybe they plan on going to China one day! You'll never know. I really like Chinese and I am going to China one day. My dream is to become a Chinese translator and translate lyrics from Mandarin-English/English-Mandarin so other people can enjoy international music.