There's a classic joke along the lines of, "Isn't it romantic seeing all those hearts carved on trees commemorating two people in love? But why do so many people bring knives on dates?"
First off, I bet a lot of heart graffiti isn't done on dates. Even so, I carry a knife with me at all times. Two actually. They're on my Leatherman. A fuckton of people carry either multitools or knives because knives are handy as fuck. Yes, like guns, knives can injure or kill people. Unlike guns, to injure or kill is not a knife's only or indeed even main purpose (unless you're some special-forces black-ops CHUD who does shit like that). They're useful tools. Humans have been using knives for thousands of years and a date is no reason not to be prepared.
So the next time you see a heart carved in a tree, don't ask why people bring knives on dates. Ask why they felt the need to damage a helpless tree instead of doing the right thing and carving their names into a cop's tire instead.