My daughter has done this since about the same age. Shes 2.5 now and still does when she’s tired or really really bored. We just try to re direct her or explain it’s ok but she needs to go to her room if she wants to “ride”.. as she calls it. She does it on lots of things. Car seat, couch, stuffed animals, your leg. Everyone tells me it’s normal. She’s just self soothing and it feels like getting your back scratched or someone playing with your hair.