I gotta be honest, this song is crazy accurate when you're in love. When you meet your s/o, all you want to do is learn everything about them, the good, the bad, the in-between, and you never stoping loving them, no matter what you find. The line "I've spent 10000 hours, and I'd spend 10000 more, if that's what it takes to learn that sweetheart of yours" can be seen as a promise so to speak that you're always going to find new things about your s/o so, instead of asking them to change, you keep putting time and dedication into your relationship, and instead of forcing your way through this, you slowly but surely learn everything about them.