Why did he moan?

Okay so I know the question title sounds weird so let me explain further. I've gone down on my boyfriend and it's understandble why he would moan when I lick, suck and play with his balls as I'm giving him pleasure.

He has gone down on me as well, ergo he's giving and I'm receving the pleasure. Now the latest time when he went down on me and licked around and I let out moan, slightly grinding. He began moaning a bit to, obviously not to much but from time to time. However I don't understand that seeing as he's not getting any pleasure. Can someone explain?
Why did he moan?
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