Well throughout the song he is describing all the good things that could happen to them in the future, and how amazing life could be, in five years time.
In the fourth verse, he makes it clear that he does not think any of this is going to happen, but even though it's all "in his head" he is still thinking about the life that they could have.
The next chorus is him saying that even if they do not know each other in the future, she will still go on to have a brilliant life, full of "love".
The final line before the last chorus "You might just prove me wrong" is saying that maybe it will all happen, and there's a slight chance that he might be proved wrong in his prediction that it really is only fantasy.
So when people say oh it's optimistic or it's pessimistic, it's neither. He has accepted that he will probably never live all these moments, and he's accepted that, and is perfectly happy just to live in the dream of his own mind.
Well throughout the song he is describing all the good things that could happen to them in the future, and how amazing life could be, in five years time. In the fourth verse, he makes it clear that he does not think any of this is going to happen, but even though it's all "in his head" he is still thinking about the life that they could have. The next chorus is him saying that even if they do not know each other in the future, she will still go on to have a brilliant life, full of "love". The final line before the last chorus "You might just prove me wrong" is saying that maybe it will all happen, and there's a slight chance that he might be proved wrong in his prediction that it really is only fantasy. So when people say oh it's optimistic or it's pessimistic, it's neither. He has accepted that he will probably never live all these moments, and he's accepted that, and is perfectly happy just to live in the dream of his own mind.