People are scum. Everywhere, everyone, all the time. As much as we love to aspire to higher standing, we are animals at heart, and the vast, vast majority of the population let their bestial instincts dominate most of their lives, often when they're not aware of it. I'll grant that there are a select few out there who are aware of this, and even fewer still who have overcome it, but assuming that this person you've just met is a waste of oxygen and changing your opinion of them as you come to understand the way they live their life is a huge timesaver compared to assuming all people are fundamentally decent and being consistently disappointed.
Edit: Also, thongs aren't sexy. They look like bum floss. French knickers and full-arsed cotton panties are the hottest things a girl can wear, apart from a pair of your boxers. It's like unwrapping a Christmas present.