This really got me thinking. We love The Princess Bride, and I knew that Wesley really loved Buttercup, but I didn't stop to think about it this deep before.
In the end, I think it's his selflessness. In the book, Buttercup admits: "All I've ever done is scorn you and degrade you and taunt you," but Wesley stuck around. If he was only doing this for himself, he wouldn't have stayed. He would have said: "okay, beautiful or not, she's not worth this! I'm outta here!"
Instead, he gave himself as a gift to her. When he finally told her that he loved her, he told her about everything else he'd done (stayed on the farm, taught himself other languages, worked out to get strong) all for her sake. He'd even been preparing himself for a grueling new lifestyle so that, if she said "yes," he could go earn enough to support them and their future family.
Wesley's selfless love is also contrasted with the "love" of another character, who wants Buttercup for his own sake... but I'll leave that for you guys to find out. If you haven't read the book+watched the movie, you should. It's hilarious.