Went to the school gym today, used the showers afterward. Dude in the shower across from me had like 3/4ths of a boner 
Suggestions from a chronic shower masturbator:
- The fabric felt good, gave him a half mast
- He knows you're looking and gets off on voyeurism
- He probably wants to jack off super badly
- Sometimes happens when lathering up, can't be helped
My suggestion: You're a homo, and wanted to suck his meaty cock.