Like many writers, I struggle with first sentences. I grew up reading old novels, so I tend to write openings with lavish description. However I recently noticed that I no longer like a lot of description in the beginning of a story I'm reading. I just want to know if the story is worth my time. So as a writer, I need to be aware of what will put my reader at ease and draw them into the story.
Here's a metaphor I came up with to help me with this: Your opening sentence is the equivalent of you standing at the front door of your house (aka your novel) and inviting the reader inside as you open the door.
Introducing the main character right away is like introducing them to the most popular guest at the party, thereby making them feel like they have someone to mingle with. I'm not suggesting that you begin with, "Princess Babbletart lived in the Kingdom of Chatter, had long pink hair, and loved ponies." But the presence of a person in the first sentence gives the reader someone to latch on to.
Let's consider a few other, less comfortable party scenarios...
Open with a long, detailed description = No one is at the party yet. It's just an empty room with no music, snacks, no drinks. The reader is the first person there and doesn't know any of the guests who were invited. How would you feel in this scenario? I would make up an excuse, shut the door, and hurry home.
Open with dialogue = One of your guests is standing in the doorway talking at your reader before she has even been introduced to anyone. It's awkward and rude. I'd be angry and avoid that person the rest of the night--let's hope it's not your main character.
Open with lots of action = Your guests are dancing on the coffee table, swinging from the chandelier, and doing body shots off your cat. Your reader is completely stone cold sober. Totally awkward. I'd run home and hide.