You can freely discuss when, where, and how many friends you were with when you lost your ultra-virginity. The only interesting disease to contract during trail running is insanity. It isn't embarrassing when people find out you train alot on your own. If you need to pee or vomit you can just go off to the side of the trail. It confuses the hell out of jealous spouses who can't figure out what they are jealous of and why it is so much fun.
Nobody tells bad trail running jokes on prime time television.
You don't have to hide your copies of Running Wild and UltraRunning. You can plan for trail runs months in advance. You can expect to get in at least four trail runs in a year. It's good even if your running partner goes too fast for you.