[chryuop]

I assume it is because you spoke Japanese since you were born. You got a different feeling towards the language, and by feeling I don't mean you like it while I don't like it. In you ears you can tell what feels/sounds right and what doesn't. A foreigner needs a different approach to the language coz we don't have those way of feeling it.
You give us a particle and tell us what it translate into and that is what it is to us. You hear a particle and you don't think what it translate into, but it creates a certain feeling inside of you that tells you what it is.
This is why when we learn grammar we need some sort of schemes that makes it easy for us to follow the right path.

Just try to think back my poor try of speaking in a more colloquial way, a total failure. We students don't have the same approach towards your language as a native speaker does. Trust me, I have lived in the USA for 10 years and still haven't fully developed the same "feeling" that a native speaker has for English. I don't think I will ever master English like I do with my language. Not because I know my language grammar better, but because I belong to that culture fully.