Girls why do you think my girlfriend LOVES her belly being rubbed so much?

I love cuddling with my girl and she loves it. Aside she loves my rubbing her belly while we make out. I don't mind it cause that is the most I'll get to touch her till we are married but I'm curious why she likes it so much. Especially when I can really easily tickle her belly button?

When asked she says that she does not know.
Girls why do you think my girlfriend LOVES her belly being rubbed so much?
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