It's a developmental thing. Kids that age are still developing logical thinking and reasoning skills, and don't have the ability (or the words) to really explain things such as feelings or their own behaviour. It will come
+1 to that.

I remember DS being 3ish (he was in 3yo kinder) and my having no idea what was wrong with him when he felt sick because he couldn't tell me.
He managed to say his tummy hurt once and I said what does it feel like? "Chocolate".

It will come with time.