We humans only pee on trees if we're a dude and we're either camping or desperately have to go and there's no one around...that is until those nuns, school children, stay-at-home moms, elderly women, and every girl you ever found attractive turn the corner. Great, the one time I couldn't find a bathroom. This is the only time I've done this, I swear. I don't pee on trees often. Really! Oh hi, mom. Shit.
But dogs pee on trees all the time, in the middle of major metropolitan areas, in front of scores and scores of judgemental humans, and they don't care. They don't give a good (or bad) goddamn. They just just keep pissing on that tree like they're in an Olympic urination competition, which fortunately doesn't actually exist.
(flickr photo Marking territory by Scarleth White; http://www.flickr.com/photos/iloveblue/2415839655/)