This really comes down to honne and tatemae. It's hard for Japanese to let anyone in because of the fear for disturbing harmony. Remember, historically speaking, Japan has been a caste-based society with complex rules of etiquette that seek to preserve social harmony and order. These things create a sense of rank and place and sometimes letting the fur fly rocks the boat far too much for their comfort.
I've seen two interesting reactions of Japanese who come to study in the U.S. One is a slow, gradual opening up, realizing that Americans also have honne and tatemae, but that they are expressed differently, and that we are much more prone to drop tatemae and let our actual opinions emerge at a moment's notice. These Japanese are usually still reserved, and sometimes uncomfortable with the strange, chaotic mess of conflicting opinions, and oftentimes return to Japan with a sense of comfort in the Confucian ideal.
The other is the Japanese student that, after a period of culture shock (sometimes severe), completely drops tatemae altogether and embraces Western expression. These run the gamut (based on personality) from really laid-back to absolutely manic about assimilation. I knew one girl who "went native" and is beating frantically at getting American citizenship because she desperately wants to be an American, hates Japanese society and never wants to go back, but she's an extreme case. Most of those who get comfortable with Western expression make friends fast, learn to be vocal with their opinions, feel comfortable with others' opinions (even if they conflict), and often decide to stay and try to live and work in the U.S. (but not always get citizenship, as they still identify with Japan).
Making friends in Japan is going to be a lot of the former, because acting opinionated is going to burn a lot of bridges, even if you don't mean to do so. If you want to make friends, remember that everybody exists in a power-structure, and it helps if you figure out what your rank is and behave accordingly. Although you are gaijin and outside the system, it nevertheless helps to find a spot in the pecking order, because it helps them relate to you more effectively. And be patient.
And alcohol gives people free tickets to honesty. A lot of people become all honne when inebriated.
Keep in mind, however, that I'm speaking from a second-hand position and don't live (and haven't yet lived) in Japan, so take it with a grain of salt.