SAM: Leave him alone, Miss Ilsa. You're bad luck to him.
While Casablanca is a love story, it's usually Rick and Ilsa that get all the attention in that department. But let's not overlook Sam's love for his friend either (a love that goes both ways, mind you). Here he is looking out for Rick, being a good wingman. He knows the emotional distress that's sure to come from another encounter between the two, and is doing his best to prevent it from happening. He probably knows it's hopeless, but at least he tries.
RICK: I remember every detail. The Germans wore gray, you wore blue.
What's a surer sign you've been harboring loving feelings for a person than that you remember every detail of your time with them? The clothes they had on, the way they were wearing their hair, their habit of talking with their mouth full…
Rick's clearly a smitten kitten. You don't recall what just anyone was wearing on a random day years in the past.
RICK: Here's looking at you, kid.
This famous line is repeated several times in the film, and it's apparent that this is/was Rick's little term of endearment for Ilsa. It's a bit more of a mouthful than a simple "sweetie pie," but it has the advantage of sounding silky smooth in addition to being sweet. Which is totally Rick's M.O.
RICK: If that plane leaves the ground and you're not with him, you'll regret it. […] Maybe not today, maybe not tomorrow, but soon, and for the rest of your life.
Rick may be talking about Ilsa, but you have to think he's talking to himself a bit, too. Obviously, he'd prefer to spend the rest of eternity with the love of his life. But if he were the one responsible for keeping her in Casablanca while her husband continued his admirable work out there in the world alone…he could never forgive himself. Letting her go is the grandest show of love he can muster. Woody Allen totally borrowed this exchange for the conclusion of Play It Again, Sam (see our "Best of the Web" section).
RICK: We'll always have Paris.
If the city of Paris wasn't considered an icon of romance before this movie, it sure has become one since. Of course, Rick isn't just talking about a physical location. The Arc de Triomphe is nice and all, but it's doubtful he's reminiscing about a guided tour. Here, "Paris" is the moment they shared together, when everything was exciting and uncomplicated. No matter what's tearing them apart at the moment, they'll always be able to look back fondly on their memories even if they can't go back there again. C'est la vie.