Why did she lay her head on my shoulder?

I was chilling in my roommates gfs dorm room and sitting on a bed next to the door and my friends girlfriend volleyball teammate who im friends with randomly came up behind me while i was sitting and the girl wrapped her arm around my back and laid her head on my shoulder. then she went to hug the other girl and left to hang with the bball team playing fortnite after that , then my roommates girlfriend was asking me if i had my first kiss and how old was i when it happened and then asked how many gfs I've had.

Why did she lay her head on my shoulder?
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