Why touch people just because you can?

Straight from the top, why do guys just touch ladies, purposefully where there's exposed skin, or just anywhere when out partying and such?

I was recently out with some friends, a small group, both ladies and guys. We got there a bit late so most people were already drunk and dancing the night away. I'm not the heavy drinker, and I don't dance too much, so I tend to sit back, vibe to the music and people watch.

I continuously see guys, mostly drunk, passing by ladies, touching and holding them, seeing the ladies recoil slightly, some welcoming it. I sit there and wonder why someone would do that, invade someone's private space uninvited.

Honestly, why would anyone do this? Sure, we have bad characters, but are they that numerous, or is it simply concentrated around clubs?