What makes a love story sing?
It's not the kissing moments or those passionate fights in the rain or the slow build to some hot action (though, I'll admit, I'm a sucker for them all).
It's the sacrifice.
It's the stepping back of one character for the good of another.
*This* is love.
No strings attached.
Not about what the character can get, but what they can give.
Their self-sacrifice is, ironically enough, what makes the character truly worthy of the love at all, and this speaks to us, as humans and readers, in a very real, very visceral way.
So, if your love story is missing that extra "zing," find a way for your protagonist to practice a little self-sacrifice.
It's sure to go a long way.