Er, well Jikanu, apparently humans can't. They just can't accept the fact humans are different? Why? How should we know? =o= Everybody thinks differently.
Sooooooo...
if you want everybody to think the sammmeeeee...
1) mass purge everybody.
2) educate everybody to think the same way. For those who don't agree, make them change. Or kill them.
4) genetically modify everyone.
There's bound to be somebody out there who doesn't like somebody else. In fact, it might even be a tradition or religious. There's going to be a whole lot of bloodshed in the world if you told them to stop hating each other. Probably more than now.
>edit
Like what Yosei said.