When you begin writing, using first person often seems natural. I struggled for a little while to get used to writing in the third person, but it's now my preferred form. I wanted to write in third because I found it harder to write characters who weren't me when I wrote 'I did this', or 'I did that'. I later found out that third person is more popular with readers and it's the most flexible form to use, so there are practical reasons for choosing third.
But there are plenty of reasons for choosing first too. It's easier to capture a character's voice when you're writing in first, as everything has to be from their point of view. Voice is particularly important in children and young adult fiction, where readers are less interested in areas such as style. They want to feel that they are experiencing those things, living that life. Having said that, many favourite characters, including Harry Potter, are written in third.
First person means that the reader only knows what the narrator chooses to tell them. This allows scope for writing unreliable narrators, where part of the pleasure for readers is discovering that the narrator is fooling them (and also sometimes themselves).
Someone was recently quoted on radio as saying first person is essentially a lie as the narrator, by definition, has already experienced those events. That can be part of the unreliable narrator, but sometimes the sense that the character is telling their story to the reader is part of the pleasure - I love the Mary Stewart trilogy about King Arthur which is narrated by Merlin. Part of my enjoyment is in the way Merlin explains what's going on, and gives background that would be otherwise be hard to digest. The books simply wouldn't be as good in the third person.
First person is also useful when there's an outsider narrator - Dr Watson, for example, or Nick Carraway in The Great Gatsby. Dr Watson allows Sherlock Holmes to be inscrutable and brilliant without alienating readers by his arrogance, Nick Carraway observes the entwined relationships between Daisy, Gatsby and Tom in a dispassionate way that would be impossible to write about from any of the main characters view point.
So there are lots of reasons why you might choose first person. I'm sticking to third - I don't want to slip into autobiography. But that's my point of view. What's yours?