At the start of Cata he did have the wise behind him. At the end of cata, death wing was no more and the horde and the ally started fighting again. He got a few new advisors to help him out, most being angry orc types that would do whatever he said. He became more obsessed with battle and across MoP devolved from what Thrall tried to teach him to be. He believed org should be occupied by the strong, so he pushed everyone that wasn't Orc / some Tauren to the slums, except for some goblins because he loves those tech upgrades. Certain leaders didn't like that, so Cairne challenged him to a duel! While he meant it to be fair, the bad Tauren from around Thunder Bluff poisoned the blade Garrosh used, so he accidentally killed Cairne in a dishonorable way, pissing off people even more.
Sylvanas decided to hire the Valkyre since they'd been unemployed since we killed the Liche King. She was super happy she could make more people, and to impress Garrosh she killed a bunch more people and rezzed a nice sized army as a tribute so she could prove she's be useful in Eastern Kingdoms. He was not happy. He put a big angry Orc there to watch her to make sure she played by his rules.
Vol'jin, still begrudgingly trusting thralls decision, gets even more irked when he Thrall pushes his people out of Org and starts using them as practice.
Currently, all races except 'pure orcs' hate him, and this is his New Horde!
Queue MoP, which starts out with him really angry and killing one of his advisors. HE will rule Panda land. Orcs invade, ally invade, everyone gets angry. Really angry. Turns out there's an ancient race known as the Sha which feed on emotions and compound them, so we end up unleashing them. After a few patches of Garrosh hunting for new tech upgrades in Panda land, they discover the Heart of Fear, the heart of an old God sealed away by a past panda Emperor since he didn't know how to kill it. Garrosh decided fear would be a pretty useful weapon since nobody liked him, so he stole it which kills Pandas most sacred land. They started feeding off each other and Garrosh goes a bit more crazy.
All the races decide he needs to be stopped. They invade Org in a mighty siege, eventually breaking past the defenses, old friends that were loyal to the concept of a Warchief, and monstrosities that the twisted magic of the Sha created. Breaking through to his Throne, Thrall finally steps in. Unfortunately, Garrosh had already been upgraded to the patch that nerfed shamans, so he beats down Thrall and says his True Horde should rule. Queue epic boss fight of Tech, magic, and warrior skills on par with lich Kings.
After the fight, ally and horde decide on who gets to kill him. Pandas step in and say we should probably do this, so they take him away.
Garrosh realizes being angry is bad and is easily manipulated after his mind is 'cleared,' so he decides teaming up with Wrathion (Deathwing's only son, who might be good?) and a bronze time dragon to go to new draenor, which should get you up to speed with WoD cinematic. The next real chapter of Garrosh is in Nagrand, where we first met him.
TL;DR - became SUPER racist, which made everyone hate him. Hated ally more, so starts teching up to beat them since goblins are cool. Goes to a land where the main enemy feeds on negative emotions, so rage gets ragier. Finds a weapon that's Fear itself, and realizes if people don't like him they should fear him.
Sorry for spelling errors, on mobile on a train >.<