Because tonal languages are weird.因为音调系统很古怪。

Okay, that's very Anglo-centric, I know it. But I have to mention this problem because it's one of the most common complaints about learning Chinese, and it's one of the aspects of the language that westerners are notoriously bad at. Every person who tackles Chinese at first has a little trouble believing this aspect of the language. How is it possible that shùxué means "mathematics" while shūxuě means "blood transfusion", or that guòjiǎng means "you flatter me" while guǒjiàng means "fruit paste"?

By itself, this property of Chinese would be hard enough; it means that, for us non-native speakers, there is this extra, seemingly irrelevant aspect of the sound of a word that you must memorize along with the vowels and consonants. But where the real difficulty comes in is when you start to really use Chinese to express yourself. You suddenly find yourself straitjacketed -- when you say the sentence with the intonation that feels natural, the tones come out all wrong. For example, if you wish say something like "Hey, that's my water glass you're drinking out of!", and you follow your intonational instincts -- that is, to put a distinct falling tone on the first character of the word for "my" -- you will have said a kind of gibberish that may or may not be understood.

Intonation and stress habits are incredibly ingrained and second-nature. With non-tonal languages you can basically import, mutatis mutandis, your habitual ways of emphasizing, negating, stressing, and questioning. The results may be somewhat non-native but usually understandable. Not so with Chinese, where your intonational contours must always obey the tonal constraints of the specific words you've chosen. Chinese speakers, of course, can express all of the intonational subtleties available in non-tonal languages -- it's just that they do it in a way that is somewhat alien to us speakers of non-tonal languages. When you first begin using your Chinese to talk about subjects that actually matter to you, you find that it feels somewhat like trying to have a passionate argument with your hands tied behind your back -- you are suddenly robbed of some vital expressive tools you hadn't even been aware of having.