I know many stories have characters unexpectedly fall in love, so how can I do the same? In some stories it's quite obvious, like the boy annoys the girl, they eventually fall for each other. How can I be more original?
How to do it right is a million dollar question.
I'm not very much into romantic novels or movies, but I've seen so many times how the author is trying to introduce some artificial obstacles for the two characters that are pretty obviously are meant for each other. So, the story, at best, typically becomes the story of how one character falls for another and slowly wins her/his heart, and at worst, a collection of romantic cliches.
But this is not what you want to do, right?
The right way to do it, I think, is making things obscure at first, so that neither your characters, nor reader knows where this is going. A good example I may point to is a movie "As Good as It Gets". It is entertaining throughout, and we have no idea at first that the movie would end up romantic, with Jack Nicholson's and Helen Hunt's characters would fall for each other.
A movement caught my eye. I glanced to my right. A waterfall of beautiful, long, red hair met my eyes. You turned slowly and my heart missed a beat. My breath caught and I couldn't remove my gaze away from your gorgeous face. We'd been friends since we were five, we fought a lot when we met, but in that moment, it was like the first time I saw you.
I never believed in love at first sight, but if someone asked me what I felt in that moment, I would have answered, "I'm in love."
My answer to you would be to not force it. Use emotions to convey it. Maybe the girl, although she argues with the boy a lot, at times she feels shy around him and doesn't understand why. It's hard to be original, all you can do is write it in your own words.