Why did me and my sister's best friend used to sit in my bedroom and kiss, when we were kids?

Granted, this happened a long time ago now, but when I was a lad, nigh-on everytime this particular friend of my sister's come round to our house, me and her'd sneak off upstairs to my room for a kiss, and we'd sit there kissing each other on the lips for hours at a time. Now, I don't get why, because we admitted, we didn't fancy each other in the slightest- (we just saw each other as mates), so why would we sit and kiss and make out when no-one was about? I admit, everytime her lips met and maged mine, I used to get a hot, burning feeling inside, (the likes of what I'd never felt before), and tingles all over my body, but why would anyone sit and make out with a platonic friend like that? Looking back on it now, I really don't understand why we used to do that, everytime she was at my family's house, other than, probably, because it felt nice. Any idea? I'd really appreciate your angles on this one, please, guys. Much obliged! Peace and love! Be lucky! :)