Because Jake tells the truth way more than a detective with a flair for storytelling should.
Jake Peralta was cursed at birth, or at least that's what he tells everyone. Of course, he never mentions what that curse might be, and it's not like anyone ever asks but their loss because he tells the story a-ma-zing-ly. If anyone, like really anyone had ever bothered to ask he would have said that when he was born he was cursed with the inability to lie. Which, obviously is total nonsense because how many aliases has Jake constructed this week? Four? Wait no, he was Marion Grensen the sub-par botanist who collects novelty spoons as a hobby for a few hours, so it's actually five. But they don't get it, no one really does except Gina because, well, she's known for most their friendship and being Gina she's twisted it to her advantage at every opportunity. He's telling the truth. But he tells the truth in such a way that they never ever believe him. So smort.
It began before Jake could remember. Well, it was when he was born, so obviously, but it sounds way cooler that way. His mum had just brought him home, his dad was... somewhere and they had been living in a pretty dodgy apartment building filled with crazies, weirdoes, and the fortune teller next door. She didn't even have the decency to look like a proper fortune teller with incense, shawls and lots of jingly janglies. Instead she was a sensibly dressed 30-something year old with a weird sort of agelessness about her that Jake's mum could swear came from magic, but Jake secretly suspects it was botox. Anyway, when Mrs. Peralta brought him back from the hospital, obviously glowing with the pride of having birthed the most fantastic child EVER, the fortune teller noticed. Everyone in the building who had it together enough to notice dropped by with well-wishes (obviously a ploy to get in his good favours. He had the air of greatness about him even at that age), and the fortune teller was no exception. Ms. Callaghan, and wasn't that the most disappointing fortune teller name ever, knocked on the door smartly and breezed in when Mrs. Peralta opened the door with baby Jake on her hip.
"I've come with gifts" she pronounced, with too much of a business-y air for someone who sounded like a breeze that smoked forty pack a day. Airy, but kind of gritty. Mrs Peralta looked at Ms. Callaghan's empty arms and distinct lack of pockets. Ms. Callaghan simply smiled mysteriously and said,
"Oh no, not a physical gift. It's more of a gift of character, think of the fairies in Sleeping beauty if you will." Mrs Peralta patted baby Jake's back a little and replied,
"Riiight. Why?" Baby Jake cried a little, because what, just because he is exceptional in every way doesn't mean he wasn't normal, and Mrs Peralta shushed him a little as Ms. Callaghan began to dig into her very much not voluminous sleeve.
"Well," she started matter-of-factly. "I was reading the cards this morning and I was told this was the right thing to do. Also, it's been a while since I did this, and why not, right?" She pulled a card out of her sleeve and held it up to the light and began to mutter to herself.
"Uh-huh, yeah, ok, I can work with this one, if I just..." as she was saying this she whipped a sharpie out, out of where Mrs. Peralta never said so Jake just assumed it was from her boobs, and began to write on the face of the card. Mrs. Peralta cleared her throat, because while this was significantly less creepy than some of the other congratulations she had received, this was starting to get weird. Ms. Callaghan just smiled at Mrs. Peralta and made a final flourish on the card.
"Sorry, I needed to adjust some things before I got started. Now all I have to do is this..." She clapped the card between both hands and blew on it in Mrs Peralta's direction, specifically to the hip where Baby Jake sat crying. Mrs. Peralta swears up and down that there was a gust of wind, and there were no windows open Jake so none of that it must have been a windy day nonsense, and something ephemeral and red drifted forward and settled on baby Jake's head and was drawn in by his tiny nostrils and mouth. It shocked baby Jake enough that he stopped crying for a second as he blinked in infant confusion before starting up again as loud as before. Ms. Callaghan shot him a fond smile while Mrs Peralta was having a minor freak out because what had this, this charlatan woman done to her precious baby boy?
"It's not dangerous, don't worry," Ms. Callaghan assured, correctly interpreting Mrs. Peralta's vaguely angry breathing and crazy eyes. "It is simply a gift of character. Truth, to be precise. Little Jake will always tell the truth, one way or another."
Mrs Peralta wasn't necessarily a believer of magic, but what she had seen was also pretty whack so she wasn't going to immediately assume Ms. Callaghan was crazy either.
"Ok. Sure. I'm just- I'll take your word for it. I'm not going to bother asking why because this is weird, you are weird, and I'm sure whatever reasoning you have is weird too." Ms. Callaghan appeared to take no offence, they exchanged a few pleasantries and she left, and disappeared the next day. No-one else seemed to remember her, so Mrs. Peralta relegated that memory to the area of her brain labeled 'Sleep-deprived hallucinations' until Jake began to talk and she realised that he always told her the truth.
Jake learned pretty soon after he started to speak that he couldn't lie. Well, that's a lie. He could lie, but soon afterward the truth just pushed it's way out of his mouth. So he began to train. Look, he had to alright? People with such extraordinary creativity like Jake Peralta, and his extraordinary ability to get himself in trouble his mum liked to add, couldn't let a silly thing like the truth get in the way of him getting away with things. If he tipped over the milk, he'd state very pointedly that look no mum, it was Gina, no she's not here but she was and it was totally her, I was here to clean it and yes, yes it was me I'm sorry I'll go get the mop. It was an improvement on his original attempts of, "It was Giiiiiiake it was Jake it was me. Damnit!", at the very least. Soon his elaborate lies turned into elaborate stories and somewhere along the way Jake learned to twist the truth a little and also to make up kickass back stories.
It wasn't until he got into the police academy that he realised how much of a liability his 'gift' was. It was hard to do pretend scenarios because, well, isn't acting kind of lying? And if he didn't get good at acting his dreams of being a badass undercover detective were about to quickly be flushed down the toilet of failed dreams. So he practiced. He learned that the more truthful a story was, the longer he could hold it. He learned to incorporate parts of himself into all his characters, whether everyone realised it or not. And most of all, he learned to pick his battles. Why bother denying to Sarge that he couldn't do a proper pull-up, let alone a proper sit up, when he could save his energy for lying to perps to get them to confess?
The only reason he accepted the FBI undercover job was because, well, he was essentially playing himself, or a version of himself. He channeled his truths into jokes about being a cop, being a mole, and all praise to the mighty Zeus that he Piano-manned with them so early, otherwise he wouldn't have been alive to tell Santiago that he still kind of liked her, you know, Romantic Stylez. It takes all he has to not blurt out what the actually real truth is. That he kind of loves her and yes, Sophia is so, SO awesome, like you have no idea, but Santiago is also kind of awesome in a nerdy uptight kind of way and has the added bonus of not being the Hans Gruber to his John McClane when at work. Jake is just glad Sophia never asks because he also kind of loves her too and given enough time he will love her more than Amy and then the truth won't be as relationship-destroying. So he resists the urge to tape his mouth up when he's around Santiago, which is always, and pushes through with sex tape jokes and being unnecessarily truthful with Charles.
It kind of breaks his heart a little when Santiago looks at him and tells him that she's done with cops. Because for the first time ever the truth shrivels up and stops pressing up behind his lips. He knows it's probably only temporary so he uses the opportunity to leap in with a quick lie about dating criminals, and look if he swung that way and if he weren't so so very angry with him Jake would probably be all over Doug Judy because that man is his soulmate in another life so it's not actually a lie per se, and he retreats. He wouldn't be able to resist the urge when it came back. He retreats and hopes to lie another day, and another day and every day until he doesn't love Santiago anymore. But until then, he'd spill his truths everywhere else, and ignore the last thing the fortune teller said to his mother.
"Truth is on the march and nothing can stop it now."
That went in a completely different direction than I was expecting. I was kind of aiming for humor and them it sort of ended up a little depressing. Whoops. The quote is Emile Zola from Le Figaro. I'm not big on the ending but it;s 12:30 at night, I'm tired and I can;t be stuffed reading it over again