Why is he so mad that I don't wanna talk about it?

So I had a little bit of an upsetting day today but it was manageable though I was a little down. My boyfriend noticed I wasn't my usual self and asked what I was upset about. I told him it was nothing and assured him he didn't do anything and I just didn't wanna recall the memory. And he got upset and said he shouldn't have asked and left to go home. I don't get why he's mad even though my mood has nothing to do with him and now i'm even further in the dumps. Did I upset him by not telling him why I was upset? It's not like I denied being upset I just didn't wanna go into detail on the issue.

Why is he so mad that I don't wanna talk about it?
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