Why does everyone hate Toby so much?

Michael hated Toby because he worked in Human Resources. He's essentially a corporate employee that's permanently based at the Scranton branch. He's definitely not (at least from Michael's point of view) an employee of the Scranton branch, so Michael sort of views him as a combination of invader, traitor and spy.

Toby's role as a HR employee would primarily be protecting Dunder Mifflin from Michael's stupidity. Michael Scott is, by design, an absolute idiot of a character. He's totally incompetent as a manager and also has absolutely no understanding of what is and isn't appropriate for the workplace, especially when occupying a managerial position. Toby would be the one who calls him up on his inappropriate conduct, reports that conduct back to corporate, and who would be responsible for telling Michael "no" before he does things that he shouldn't.

Children will occasionally tell their parents that they hate them (or at least don't love them) when they've been denied something they want or have been told off for doing something they shouldn't. Michael's essentially having a childish - and ongoing - reaction against the authority figure who denies him the things he wants.

The impression I got was that the other employees didn't have any particularly strong feelings about Toby either way. They didn't exactly dislike him but they didn't consider him a friend either.

Michael dislikes Toby, not just because of the fact that he works for corporate and this isn't apart of Michael's work place "family", but also because Toby is the very antithesis of his personality: quiet, mature, typically sticking to the rules, etc.

The episode "Casino Night" has a pretty good quote from Michael himself after Toby points out how inappropriate Boy Scouts being at their casino would be:

Why are you the way that you are? Honestly, every time I try to do something fun and exciting you make it not that way.

There's a common theme with the Toby hate, and I'm quite convinced that it is on purpose. It is almost always Michael who is mad at Toby and the things he does.

There are exceptions to that rule; and those exceptions are consistent.

When Jim becomes a co-manager, we see a few times where he loses his cool and gets mad at Toby, and at one point agrees with Toby being "the worst"; from what I remember. When Dwight becomes manager, he also has problems with Toby. When David Wallace is talking about the corporate office to someone (pretty sure it's Jim), he mentions that everyone is great "except" their HR guy, Kendall.

There is a reoccurring theme in The Office where management and HR do not get along.

I think it's actually quite simple:

• Michael basically loves all of the staff, and sees himself as a super-great and nice person, so hating Toby for no reason is a strong gag

It may also have to do with

• Everyone at the branch work for Michael, except Toby, who works for corporate

Thus, Toby is outside of Michael's direct chain-of-command, but is a presence in the office, which functions partly as Michael's personal social club.

• Michael does not have direct power over Toby

It's obvious from these comments and the show why Michael hates Toby.

If you pay attention to his actions however, you'll find that Toby is actually a horrible, petty person. He's one of those "nice" guys that seems shy but is actually very selfish. He wants to blow off going to his daughter's play to go to Pam's art show. Also when he wins a stuffed animal and wants to give it to Pam, and Pam has to remind him that he has a daughter he could give it to, it becomes clear that he makes selfish choices. When Jim and Pam are together for the first time he is also really rude about it with the memo about PDA at the office and saying they don't have to disclose their relationship because it's not serious enough.

If Toby would be more confident and outgoing, he would be worse than Packer.