Why does a guy refuse to open up?

Well I know that guy for months now and we talk about everything and nothing just random things, but that one time I asked him about importance of people in his life, he told me that I may not have that much importance in his life that I have him in mine, and it is obviously for some REASONS that he didn't want to tell me that night exaclty because he doesn't want to reveal himself all at once.. I understand.
Also told me that he couldn't talk 2 hours straight with me only, (did not offend me it's pretty normal), because he will start talking about some serious things.. I wasn't really following him here. He said he didn't want to talk about everything in just one night, but personally I started thinking they are just excuses. I wanna figure out what type of guy is he and also don't wanna be annoying or clingy to him but make him feel more conmfotable with, do you know what I sould do?
Why does a guy refuse to open up?
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