Have you ever had those moments that someone asked you if you liked your best friend's crush? If you haven't, you're lucky. If you have, you probably answered something along the lines of "No, that person's just a friend!" Hmm, does that phrase sound fur-miliar? (Spoilers for Animan). In late season one, it was revealed that Nino had a crush on Marinette. As his best friend, Adrien knew about this and tries to help him get the girl--while being oblivious to Marinette's crush on Adrien. This got me to thinking, was the reason that Adrien called her "just a friend" in season one Nino? This is what we know for sure, Nino had a crush on Marinette. Adrien tried to help Nino. They failed and Ladybug locked Nino and Alya in a cage. Nino and Alya became canon (the best ship in the series, but that's just my opinion). So, since Adrien knew about this crush, it is plausible to say he put the pal before the gal. It also makes sense since he noticed Marinette's crush on him more in season two. Maybe that could be because since Nino had Alya, it was ok to look at Marinette a different way. He wouldn't be breaking the bro code.
Just Saying, -Jane.