Harry Potter, Boy-Who-Lived, Savior of the Wizarding World, was thoroughly sick of England.
His parents were murdered, he was stuck with magic-hating muggles, and not one resident of the oh-so-proper community of Little Whigning, Surrey, cared one whit about the waif-like little boy in the cupboard-under-the-stairs. He was OK with that, he survived that, because at the time he didn't know life could be any different.
But then Hogwarts came with its marbled halls and hope for the future and Magic, and he allowed himself to get carried away until his very first lesson with an abusive bastard of a potions professor brought him crashing back down. From then on Hogwarts was distrust, and awful teachers and fear, with life threatening situations year after year and no one ever listening to him but that was alright. He was OK because he had Friends.
And then there was the war. Friends and acquaintances and loved ones were dying left and right, with heartbreak and betrayal around every corner. He was hunted, Undesirable No. 1, and the hopes and dreams of every decent magical Brit and the fate of the world rested on his shoulders. Still he trudged on, he was OK, because he had to be, it was his Duty.
Now the war was over, his duty done and his friends dead. Hogwarts wouldn't re-open for some time, and even if he could, Harry wouldn't go back there. It had been a long time since he could shield himself from injustice with ignorance, so he knew what the press was doing was wrong, holding him up as a Golden Idol, practically begging the masses to make him fall.
So he would leave. Harry James Potter was his own man, he was Thoroughly Sick of England, and he had a 5:45 ticket out of Heathrow. It was about time he started being more than just OK.