"He has always wanted to cook for me."
Translation:Han har alltid velat laga mat åt mig.
You can't really say laga mat för mig – or, that could possibly mean 'with me as his teacher' or something like that. The prepositions that work are till, which only means that 'he will cook the food that I will eat' and åt which can mean either this or that he cooks it so I don't have to.
Oh okay, thanks. I think there was a multiple choice question, though, offering both "för" and "åt", and I picked both of them, which was deemed correct by Duo. This is what led me to my assumption in the first place. Shouldn't "för" be considered wrong then, seeing as it has a totally different meaning?