I think one of the fun aspects of 'opposites attract' ships is that, at their cores, they quite often end up being far more similar than they expect/would like to admit!
Sancho appears to be a pessimist bordering on nihilism because of her lonely past, wherein she had literally no one until Don Quixote swooped in and made her into his Kindred. But even when she had a family, the main pleasures she sought out were rooted in their connection to her sire. And she hides her true enthusiasm about things for various reasons, among them likely insecurity about being openly happy considering said past.
And if Meursault is anything like his literary counterpart, he may have also had a mother who he held a connection to and whose death affected him greatly. He seeks out simple pleasures because those are just what makes him happy. He's clearly not emotionless, just suppresses his own thoughts and ideas a lot because, as he says, most people don't really listen to him in full because when he Does talk, he has so much to say that it puts people off.
In the end, they both appear to be people who put up different facades, one of overzealotry and one of indifference, because they believe that their true feelings wouldn't really be understood by others.