Disclaimer: Not mine
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Max knows everyone still sees him as a kid, but he's grown up enough to realize his feelings for Danny... and that they're nothing compared to what Danny feels for Kendall.
He wishes he could be immature as everyone thinks he is about this, that he's still naive enough to believe that he could just tell Danny how he felt, and not only would Danny be okay with it, it would turn out he felt the same all this time, and then he'd kiss him and he'd never have to hear about Kendall ever again... but he knows better. He's seen the way Danny's face lights up when he remembers her.
And he knows he can never compete with that.
It... it's taken awhile, but Max has grown up, when it comes to Danny at least. He knows Kendall makes Danny happy, and Danny being happy is all that Max really needs (it's not what he wants, but the difference between an adult and a child is that the adult realizes that needs and wants aren't the same two thing). So he'll handle things maturely, show no signs of a broken heart when Danny finally works up the nerve to seriously pursue Kendall, and never let any of his own feelings overflow into their everyday lives.
He can do this. He's not a kid anymore.