I would say this song is basically about unrequited love: 'I really don't think you could know that I'm in heaven when you smile'; The guy doesn't see how the girl can't feel the same way (i.e. 'think like [he does]'), and would hope to turn her around to his point of view ('I always thought I'd have the answer', 'I might just have the key'), but he's pretty sure it's all in vain, 'no matter how hard [he tries]' to think otherwise.
I would say this song is basically about unrequited love: 'I really don't think you could know that I'm in heaven when you smile'; The guy doesn't see how the girl can't feel the same way (i.e. 'think like [he does]'), and would hope to turn her around to his point of view ('I always thought I'd have the answer', 'I might just have the key'), but he's pretty sure it's all in vain, 'no matter how hard [he tries]' to think otherwise.