Why does he get off of me then?

So recently my boyfriend and I discovered laying down and making out. Now, he likes to be on top of me, but not straddling, while me kiss. He will get off of me in the middle of making out, so it is pretty intense/passionate. It has only happened these last two times. I don't know if maybe he is getting used to it or what. Why is he doing this? Oh, and I plan to ask him next time, but, I keep forgetting to ask him until he has left because I am just so excited and so happy that he is with me. It just kind of mystifies me when I recap the evening I spent with him. Thanks!