You have to mean it when you say it and you better mean it because telling this lie would be no less than a criminal offense. It is one of the most important things you have to tell him/her, not because it will get you more, longer kisses but because it will make them feel good.
“I think I can conquer the world with you by my side”
You always heard people say how love can conquer the world and climb every mountain and now that you stand holding his/her hand, you realize that it is possible and you tell your partner that. It may be, from a realistic point of view, impossible but who cares until it makes you two feel like you are on top of the world?
“I would die for you”
This is something they would tell you to shut up about because it involves you dying but, nevertheless, it will come out as romantic. It means something to say it because you have reached a point where you would consider putting your life at stake for their safety and that is, without a doubt, pure love.