Why is Toby Flenderson the way he is?

According to some fan theories Toby is the Scranton Strangler.

It's never "proven" but there is some evidence that points to Toby being a serial killer:

His wife left him, he struggles to connect with his daughter, his boss treats him like crap, and he works at a dead-end job in a dying rust-belt city. Every attempt at humor, or even general conversation, is hijacked and destroyed by Michael. Toby clearly has a lot to say, but is never able to say it.

In season three, upon learning Pam is single, Toby attempts to win her over, but fails.... In season six, Andy, as a gift to Pam and Jim, frames the newspaper from the day CeCe was born. The headline? “Scranton Strangler Strikes Again.” The joke is meant to be that the headline is morbid and an unfitting commemoration to a baby being born, but this is the first clue. Already, we’ve seen the connection between lust for Pam and Toby gripping things tightly. Maybe the idea of the woman he loves having another man’s child pushed Toby over the edge?

After getting back from Costa Rica, Toby is selected to the jury in the case of the Strangler, which leaves him ecstatic. He claims that he’s just happy that people care what he think. In reality, he feels validated. This is the first time Toby has ever seemed truly happy, and the cause is related to the strangler. His plan worked! After all the years of being ignored and deemed irrelevant, Toby’s actions as the strangler have made people care about him...

...All of this culminates in season nine, when Oscar admits that the numbers he handed in to Toby were false and he simply did it because he wanted to frame Kevin. Toby suddenly becomes worried and admits to Oscar that when he was on the Scranton Strangler jury he was pressured to convict him despite believing he may have been innocent. Toby is not guilty because he thinks the Strangler may be innocent, but because he knows the Strangler is taking the fall for him. Eaten away by guilt, he visits the Strangler in jail, but the cameras are not allowed to follow... Toby confesses to the murders to try to relieve his guilt, and is strangled by the innocent man who is taking the blame.

More subtle clues:

Toby calls asbestos “the silent killer,” to which Michael responds, “YOU’RE the silent killer.” Toby glances at the camera and says “you’ll see...”

And

On the day when the falsely-accused strangler is being pursued by the cops, the staff is sitting at Toby’s desk when his phone rings.

But where is Toby?

See these videos for more:

Video: Toby is the Scranton Strangler (The Office Theory)

Video: Toby is the Scraton Strangler - Explained and Proven

I 'MAY' be able to answer your question.

Let me be clear, everyone makes a valid point here, and as stated before, we don't exactly see enough background or private moments of Toby's life to be 100% sure.

However, there is one episode that stands out to me. I don't know exactly what season or episode it is, but it's when everyone finds out that Michael is moving to Colorado. Toby has a video chat with his brother Rory Flenderson, and we find out that they both have very similar ways about them. They both are dry, boring, and just plain uninteresting.

To me, this is a joke indicating that some, if not all of Toby's immediate family are all the same, likeit's some kind of family trait or genetic.

But as always, I may be wrong...

The Office is, by design, a very quirky show where the majority of characters on the show are "characters". I don't want to say most, but 60% of the regulars have strong personality quirks. The show is meant to be a satire on how crazy the people you work with are.

Michael Scott - A child in a man's body. He's lovable, but selfish, and he's completely 100% out of touch in a lot of ways. He's not aware enough to not crack sex jokes at a sexual harassment seminar, and he considers taking the entire office out to watch him and Dwight spar is a good use of company time. There's a little Michael Scott in all of us -- that's why the character works.

Dwight - crazy/bonkers/loony, but believable.

Kevin - really slow. "Want to see how many M&Ms I can fit in my mouth?"

Angela - crazy, anal, cat-lady, spiteful.

Phyllis - Pretty normal, brags about her rich husband a bit. Also fat. Fat isn't really a personality trait, but there's 3 really fat people on the show, which is a little unusual for prime-time TV.

Creed - psycho.

Meredith - alcoholic

Oscar - normal but gay (not that there's anything wrong with that)

Ryan (BJ Novak) - he's close to normal, a little bit of a superiority thing going, also, ends up a drug addict and in jail for fraud.

Mindy - Super clingy and nuts for shopping.

Andy Bernard - not sure how to describe him, but he's crazy too. Weird personality.

So, the only ones who come across as normal to me are Jim, Pam, Phyllis (close enough), Stanley (who's very fat), Darryl and Erin, and Erin strikes me as a little weird, but not enough to make the crazy grade.

That's 6 regulars who are what I might call normal and 9 regulars with distinct personality quirks.

So, the show leans towards personality quirks and Toby has a personality quirk too. He never has fun, he never smiles, his voice never shows any emotion. He's as interesting as dry toast and on top of that, he hits on Pam (weird and so not an HR thing to do). And when he hits on her, he comes across as even more boring and sad than usual. Toby's obviously not a major character, but like many of the personalities on the show, he's a flawed character. He's the way he is because the show is built around crazy and flawed personalities. The show is rather unique in that, all that flaw comes across as mostly funny and all the characters (possible exception being Creed) come across as believable. Parks and Recreation has the same model where the majority of the characters are, in some way, crazy, but still mostly believable. Though I think Parks and Rec sacrifices some reality in exchange for comedy, while The Office tries to keep all its characters believable.

But that's why Toby is the way he is, because having a personality flaw is a requirement for many of the characters on the show.

Now as to backstory, which could also be an interpretation of your question. The Office never got into backstory. We never learn why the characters are the way they are, except maybe Dwight who grew up on a farm with a crazy brother and presumably crazy parents. But for the most part the characters are just the way they are and for virtually all of them, there's no history or reason to say why they are the way they are.

Not sure that answers the question but I figured I'd give it a shot.