Why won't he tell me why he cheated?

My boyfriend left his email up on my computer. I snooped looking for this embarrassing/funny email he said he had sent the other day. Found out he had cheated on me with another girl the day after we celebrated his birthday 3 weeks ago. I had taken him out for dinner and bought him expensive gifts. When I confronted him about it, all he said is that I never should have invaded his privacy. When I asked why he did it, or if he was even sorry, he wouldn't give me an answer. I broke it off, but I hate the lack of closure. We had been open about everything, he had even told me the day before I found all this out that I was the best thing that happened to him. Now I am left without any answers and just horribly confused. If the sex was great, communication was great, then why would he do this?
Why won't he tell me why he cheated?
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