When you think about him. You see a short flashback of him that turns you on instantly and primally. Like chills, rock hard nipples and gasping for air suddenly. Feeling that "startled" jolt of scary excitement you get when you unexpectedly run into your crush.

For me I used to picture the way he would grunt when he was cumming. He was so hot, strong, reckless, and manly. Someone you have the biggest crush on. The BIGGEST crush you've ever had in your life. You think about him constantly and obsessively. All you want to do is spend time with him. 

Fuck I'm never doing that again.