Random boner while hanging out with my friend
I'm an 18 year old dude and yesterday I was hanging out with one of my closest friends (a girl) who I've known for years at the park. As we were starting to head back we noticed the playground was completely empty, so we thought just for fun we'd have a quick play there. So we climbed up on the structure and we were goofing around on a clatter bridge. At one point she crawled past me and as she did her head brushed against my butt, at which point I got a boner. Thankfully she didn't notice and we continued to play as normal, then went home. But I really don't understand why it happened then since we have hugged each other and had other physical contact and it's never happened then. So why did it happen this time?