For the same reason that it's weird to wear a bulletproof vest everywhere you go: it makes life less lively and enjoyable and it makes human contact less fulfilling. Your focus becomes staying alive rather than being alive. You lose out on social serendipity. You rob yourself of fulfilling experiences like dancing with someone you just met, singing karaoke with strangers, smiling at a boy or girl you're into getting smiled back at, smelling the roses, and taking in the here and the now.

You can look behind every bush for a killer, maybe it will save your life, but it's no way to live life.