In touch with his emotion

Just since about last week, Sean has mentioned several times about how he felt.

The first time was last week, when I took his mini tomato and throw it to the rabbit. His demeanor immediately changed and he looked truly upset. So I asked:

me: What's wrong? Are you angry?
Sean : (nods) Yes, Sean Sean angry.
me: Was it because of the tomato?
Sean: Yes
me: Oh, I'm very sorry.

I knew he didn't like the tomato because the night before he spit it out, and I didn't want him to squeeze tomato juice on my white pants. I was taken aback because I didn't realized he would be so upset. Oops... my fault.

Then several times, he would tell me that he was happy. (can't remember when)
Sean: Sean Sean happy, Mummy happy, daddy happy, Akong Ama happy.
or if he did something wrong, I would tell him, "Sean, I don't like that." Then he would smile his most winning smile, squinting up at me, and say, " Mummy happy?" coaxing me to smile, and it works. =D

Then last Saturday, when Akong Ama went out, with his head down, he said
Sean: Sean Sean Sad.
me: Why? Is it because Akong Ama went out?
Sean: Yes, Akong Ama go out, Sean Sad.

Later that afternoon, we went to the library. I told him to get a book and "read" it while I look for books to borrow. He took some books, flipped through them, laughed loudly, and came to me
Sean: Sean Sean funny!

He can't really say exactly what he means, but I was just surprised by his telling me how he feels. Where did this come from?