Meursault has clearly given up on life’s meaning because he no longer has enthusiasm towards things that would make any sane person feel emotion. The entirety of the book Meursault’s expression and tone seems to just feel uninterested in the things around him. No amount of passion or happiness or anger around him would change the course of his mood at all. This is due to the fact that Meursault has engraved into his mind that life gives no meaning whatsoever from a very young age.
When Marie asks Meursault to marry her, there is an expectation that he feels some kind of emotion, whether it’s happiness or sorrow doesn’t matter as long as he resolves his overarching existential crisis. Instead, he only gets married because Marie wants to, NOT because he wants to as well. It’s such a doomed to fail relationship between the two, because while Marie would put full effort into their relationship, Meursault would just say “whatever” and continue his business. Meursault has this tendency to just give a green light to any request that comes his way. It doesn’t matter if he feels a certain way about their request, he just allows it to happen. He is oddly content with anything he disapproves of and will only subtly shake his head.