I think my favourite argument of Phinabella shippers who believe that Phineas has a crush on Isabella is that they don't speak the same love language.
Isabella's vision for romance stems from all the tropes you see in young adult fiction. The kissing in the rain. Sharing an umbrella. Cute little ice cream dates. Personal heartfelt gifts with value that stems not from grandeur, but sentimentality. Sure she fantasizes about them running away together into the sunset as Phineas turns into a centaur to marry and live happily ever after but they've been purposefully overdramaticised and she thinks they're unrealistic. She wants PHINEAS to tell her he likes her, because he's the boy.
GIRL. Phineas Flynn Only Speaks in Grandeur!!!! You need an umbrella to walk to your house in the rain? NO PROBLEM! He's built an underground tunnel fully equipped for housing between your houses in 10 minutes using a shovel. Never seen a rainbow? No problem! He'll build a contraption to shoot rainbows and disrupt global weather to make you happy. Oh shoot. You meant unicorns? No problem! He'll genetically engineer one from scratch for you right now! He's made full day plans and a singing event with chorus girls and 20 tier cake of your favourite flavour the size of your house for your birthday DONT LOOK THE REST IS A SURPRISE. IT GETS BETTER. What's your whim, Isabella? You want a face carved into the mountain? A national holiday where everyone fawns over your cuteness? A chair in the senate? What does he need to do to prove it? Walk over lava? Climb a mountain? Travel the world in 24 hours????? Girl, the ONLY thing he cant do for you is say what he means out loud! He's not good at feelings! That's Ferb's job! You were always the braver, put together one!!! For God's sake Isabella Garcia-Shapiro, don't make me slap you!! He's been trying to say the same thing this whole time!!!