This song seems to be rather miserable, despite what most people get from the title.
"...and I'll pay for sanity but sanity don't come cheap" means he'd give anything to be happy, to be normal, but he feels it's something he'll never have, or something he'll have to work harder than anyone else to have. "You don't know the truth/and I love your life," seems to me, that Johns is saying that he is jealous of people who never see the bad aspects of life, never notice the harsh truth of things, and he wishes he had their ignorance because it's better than living in the bitterness of reality, he's telling them to love their lives because they're better than his own.
This song seems to be rather miserable, despite what most people get from the title. "...and I'll pay for sanity but sanity don't come cheap" means he'd give anything to be happy, to be normal, but he feels it's something he'll never have, or something he'll have to work harder than anyone else to have. "You don't know the truth/and I love your life," seems to me, that Johns is saying that he is jealous of people who never see the bad aspects of life, never notice the harsh truth of things, and he wishes he had their ignorance because it's better than living in the bitterness of reality, he's telling them to love their lives because they're better than his own.