Guys, Why does he push my hips down when I'm sitting on his lap?

Everytime I'm with my boyfriend and I'm sitting on his lap he always pushes my hips down. He starts out gently but gets more forcefull. Normally we'll be making out while this happens if that helps at all.

I don't really know why he does it or what I should do when it happens?
Guys, Why does he push my hips down when I'm sitting on his lap?
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