Because the opening line is your first impression on a reader. It's what hooks them. Makes them curious. Makes them want to read further.
I like opening lines. I often pick up books, even if I'm not interested in reading them, just to see how they start and maybe file away some of the more unique intros for later inspiration and reference.
There's not much to this exercise. It simply involves writing five to ten opening lines. And just opening lines. Keep them simple or make them complex. Be as realistic or absurd as you like and don't worry about these lines leading to a story. (That's added pressure you don't need.) Though they certainly could. I've thrown many one liners into the morgue file for possible use in the future.
Here are a few of mine:
♣ The bed was shaking; it felt like an earthquake, but earthquakes didn't smell like fabric softener and cookies and they surely didn't whisper "Mama, Mama, Mama," over and over again.
♣ The cloud came suddenly and from the east, trailing lightning behind it the way a bride trails veils.
♣ "Have you come to kill me?"
♣ I always needed a drink after a job.
♣ Lee woke from a dream about paneer burfi and thick aromatic coffee—so real she could taste the bitter and the sweet mingling on her tongue—to the smell of decay.
Give it a try.
Share in the comments, if you're so inclined.