That's a really good question, especially since I don't exactly shy away from first person.
So, I suppose, the real question is more along the lines of "Why do I think you shouldn't use first person?"
Well, okay, it's not that I think you, the specific you sitting here reading this post, shouldn't use first person; it's that I think the general you out there shouldn't use first person. At least not until you have figured out how to write in third person. First person, especially for the beginning writer, has too many traps and short cuts; until you know how to get around them, you should write in third. And, actually, it's writing in third that will help you to learn to avoid the snares.
So let's start with descriptions.
There is what I think must be some sort of automatic desire to want to describe our characters to our readers. I suppose it's natural. But, really, how often do you describe yourself? And, more importantly, how often do you describe yourself... to yourself? I'm going to guess, here: almost never. Maybe, completely never. And, so, when I'm reading something in first person, and the protagonist starts describing what she looks like -- her sparkling green eyes or long, silky blonde hair -- I can't stop myself from groaning. I'm not even sure that incredibly vain people actually think about themselves that way. So, basically, if you have an uncontrollable desire to describe what your character looks like, write in third person.
Besides, descriptions aren't nearly as important as some people seem to think they are. For instance, in The Ocean at the End of the Lane by Neil Gaiman, we get no description of the protagonist; heck, we don't even get a name. We know he's a man home for a funeral remembering something that happened when he was a kid. That's all we need to know. Descriptions would be superfluous. And, in The Dresden Files by Jim Butcher, we never get anything more than a vague description of Harry. We know that he's tall, which he tells us in relation to things when it matters, like a tall man squeezing into a Volkswagen bug or that he can run fast because of his long legs. But he never tells us what he looks like, and he doesn't need to. Readers are quite capable of envisioning what characters look like all on their own.
Then, there are emotions.
Dealing with emotions in first person is problematic at best. It's too easy to fall for the "telling" short cut. I mean, you know how your character feels, right? And your character knows how she feels, right? So why spend a paragraph showing us how she feels when you can do it with one simple telling line: "I was mad." Or, even, "I was mad. No, I was pissed." [Which is an actual line from a book I read not too long ago.] The problem there is that it doesn't tell us anything significantly different from "I was mad." The terms are too subjective, and I've heard just as often, "No, I'm not mad; I'm just pissed." Evidently, there is no clear line as to which side of mad pissed lies.
And just saying "I was mad" or "I was sad" or "I was happy" or I was anything doesn't do anything to connect us to how the character actually feels. However, there is a significant difference between "I was mad" and:
As she stood there listening, her face turned red. It worked its way up from her neck and crept up her cheeks as if it was sneaking up on her eyes. The more red her face became, the tighter her fists clenched and the more her jaw bulged from the pressure of her teeth grinding together. If he had been paying attention, he would have seen the signs of the coming explosion, but he was too busy wallowing, so he was caught completely off guard by the verbal eruption of "You lousy bastard!" and the slap that followed it. He hadn't quite caught up to what was happening when the dishes began flying, the first plate catching him under the eye and covering him with the carefully prepared dinner.That's not really a paragraph that translates well into first person. You can't see when your face turns red and most people are, actually, completely unaware of their physical reactions to being angry, so a first person description like that is rather like the character talking about her lustrous blonde hair. At any rate, it's the showing of the anger that really allows us to connect with the extent of the anger. And the above paragraph is much different than, say:
As she stood there listening, her face turned red. It worked its way up from her neck and crept up her cheeks as if it was sneaking up on her eyes. When it got there, it spilled out of her eyes as small crystal drops that rolled down the heat of her face. Her teeth ground together and she turned and stomped out of the room. So caught up in wallowing in his own misery, he didn't notice her leave, not then, and kept right on talking.Those two paragraphs give us two very different reactions to the same statement of "I was mad" and, although you can get a little more detailed in first person than the simple statement, you can't give us the same detail, like the male character continuing on after the female character leaves.
In fact, it's all in the details.
The truth is that first person allows us much more easily to skip the details and go right to the conclusion. Things like, extrapolating from the above paragraph, "I could see that she was getting mad," or, "I knew she was hurt." Instead of showing us her rising anger. And, yes, you can skip those details in third person, too, but it's easier and more natural feeling to skip them in first.
When it comes right down to it, third person, not first, should be your default perspective from which you write. What that means is, unless there is a specific reason for using first person, you should use third person. The perspective you're writing in should serve the story and, if first person doesn't add to the story, if first person doesn't bring something to what you're doing that third can't, you shouldn't be using it. One is not as good as the other where writing is concerned, especially when everyone out there is writing in first person. Which may not seem like a fair thing to say, but when all first person voices sound the same (and they almost all do when "everyone" is doing it), there is nothing to distinguish your book from anyone else's. That's not a problem with third since, to a certain extent, you expect third person to sound the same. It's the observer's view, after all.
Remember, your best question when writing, even about something as basic as perspective (or, maybe, especially in the case of something basic), is "Why?" So, when you ask yourself the question, "Why am I writing this in first person," if your best answer is "Because it's easy," then you need to look at what you're doing. The perspective should serve the story, not the other way around.