I love this song.!
The singer is tryin to tell the girl he is in love with that she is in his heart and that if his "heart was a house she'd be home". But they're not together. Which is why he sings. "We got older and I should've known (Do you feel alive?)
That I'd feel colder when I walk alone (Oh, but you'll survive)" But the fact remains that he's still in love with her.