理由 - to Aska_Z's point usually uses in negative context also equivalent to 理由;缘故;借口. 原因 - usually used in neutral context and there's no other negative association with it.
This is an excellent question - as a native speaker I have think for a while to answer :)
2012年1月7日
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两者的意思就不同了。理由:事情的道理,根由。/原因:造成某种结果或引起另一事情发生的条件。
2011年12月19日
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I agree with Aska_Z,'理由'is more like an excuse. “理由“ is usually used some matters that u don't think it is pleasant.(or u r blaming someone and ask someone give u a explain).E.g. 你有什么理由认为我会回家?