Turned on watching my roommate masturbating...
When I was in the Air Force, the enlisted personnel were assigned to live in dorms with one roommate. Our dorm was set up with bunk beds and I slept on the top. My last year in the Air Force I roomed with a 19 year old who'd been in the service for about a year and had a girlfriend in the Midwest.
Anyway, one weekend morning, I woke up to rhythmic, rustling noises from the bottom bunk. I quietly peeked over and noticed my roommate on his stomach humping up and down. He was covered under his blanket, but I could see his butt moving in rhythm up and down. His eyes remained closed, so he didn't know I was watching.
I immediately became very aroused as I watched him humping himself. I watched him for what seemed like five minutes and he still hadn't cum! By then I was extremely horny and I couldn't wait any longer. I stopped peeking and started on myself. Oh man, I came really good. I was just so turned on watching someone else masturbating.
When I finished, I peeked again and he was still at it! I got turned on again and stroked my dick again till I came. For the next few months, I would peek over and watch him as he played with himself. I never let on that I knew, but he sure did provide me with some hot orgasms!