Am I missing something?

I feel like I'm missing something. My boyfriend and I had an argument last night. I found a bottle of medicine of his (it was in plain sight) and recognized what it was for. I don't care he has that diagnosis. I don't care what diagnoses he has. I was upset that he didn't tell me and chose not to tell me. I thought we had open communication. He said he doesn't like talking about his medical stuff because it's unimportant to him. If it was something serious like cancer though he'd tell me. But to me it's important I know these things. We've been together for a year, I knew about a couple of his diagnosis but not this one. He was upset because he never forced me to talk about anything I don't want to but he felt I forced his hand to talk about this. Am I missing something? Did I do something wrong? Please help I'm so confused