1. I think the strongest reason to choose first person is if your character has a strong voice and a unique world view.
2. It's easier to explain what a character is thinking and why they act the way they do.
3. It gives a feeling of immediacy.
4. If the book is mostly about a character's emotional journey.
5. I've heard that many YA editors prefer books to be in first person. [Do you think that's true?]
Why not write in first person?
1. You can fall into cliches. Think of all the first person detective novels about hard-boiled private eyes.
2. If your book is plot-driven, as opposed to character-driven, it might be harder to pull off.
3. If something happens to the main character that is very traumatic, first person might be too close.
Feel free to add in your own reasons.
This summer, at the request of different editors, I rewrote one book from first to third and one book from third to first. I had a middle grade book in first person, but the voice wasn't unique enough. In third person, it simply worked better. The second book was a YA thriller. My editor for that book thought it would work better if it were in first person, helping readers understand the main characters actions.
Both were absolutely right.
Here's a bit of Harley Jane Kozack's Dating Dead Men, showing some of the pros and cons of writing in first person:
It was a cigarette burn.
I could scarcely have been more shocked if I'd discovered it on my own flesh, appearing out of nowhere, like stigmata. But it wasn't on me. It was on my grass green carpet, Aisle 3, Condolences/Get Well Soon, where I knelt, rooted in horror.
"Dear God," I said. "Dear God. Dear God."
"Girl, get a grip," Fredreeq called out, barging through the front door of my shop, Wollie's Welcome! Greetings. "I can hear those 'Dear Gods' all the way out to the parking lot. Did someone die? Is it Mr Bundt? Please tell me Mr Bundt died and I can take the day off and go to the beach."
"He's not dead. He's due here any minute. I was doing the final Dustbusting, and look, look — " I waved at the carpet. "At the last inspection, Mr Bundt questioned the decor. I told him it was French Provincial. I can't pass this off as French Provincial."
"No." Fredreeq loomed over me. "Cigarette burns are strictly Trailer Park. White Trash [not a bad way of saying Wollie is white], no offense." She leaned down, sending a wave of Shalimar my way. "That's one hell of a burn. That is the mother of all cigarette burns. That's a cigar burn."
I looked up at my friend and employee, took in her attire, and said it again. "Dear God."
Earrings the size of teacups dangled from delicate earlobes. Zebra- print stockings stretched from the hem of a very short, very tight skirt to a pair of velvet stiletto heels.
"Yeah, I know, I'm pushing the envelope here." Fredreeq straightened up and moved to the cash register counter. "Is it the stockings? You think bare legs are better?"
It was a tough call. I wasn't wearing panty hose myself, but I had on a long calico skirt and socks and red high-tops. Also a red sweater with a dalmatian applique. It had seemed like a good outfit an hour earlier, but now I wasn't so sure. I'm over five foot eleven. [This description is a little strained in first person.] Next to Fredreeq, I could look like a piece of playground equipment. [I think this comment works well in first person - gives you a sense of her sense of humor.]
"Maybe," I said, and turned to scratch at the cigarette burn with my fingernail. "You're black, [this is kind of As you know, Bob] which I always think makes the high heels-no stockings look — "