Mike knew what Will was saying in that van scene.
Mike had no reason to react so strongly to finding out El needs him. He already knows that! They fought about it. Broke up. And he is eaten with the guilt of it.
When Mike is talking about what if SuperMAN doesn't need him anymore, he is talking about Will. Will, who didn't reach out after he left Mike in Hawkins. ("This past year.. has been weird." "It's been a year, Mike!" -No, boys. It was half a year).
Neither of them are able to say how they really feel directly because what if, what if the other doesn't like the truth?
When Will bares his heart with the painting, Mike goes from delighted to gulping, shuddered breath, lip glancing. He spends most of the reveal looking overwhelmed or like he's in a dreamy love haze. The only exception being when Will says the painting is from El! And then it's quickly so obvious to Mike that it isn't.
"EVEN EL. Especially El.."
Holy shit. Will was just handing him everything with that line. What Mike has to hear is: I painted this for you. You're The Heart. Without you I know we would all fall apart.
Hastily adding that "even El, especially El", would fall apart too, doesn't change that Will just directly told Mike that this is how he views him.
It makes it clear that 'The Heart' theme is primarily driven by Will, and Will just tacks El on at the end of it.... in the painting she commissioned, that she isn't in herself, in a game we never see her play, with a heart right there on his shield that has clearly never been discussed, and El never seemed at all interested in finally giving him his commissioned gift despite Will clearly carrying the painting around..
Mike's emotional reaction in the van is to Will. Just Will.
His painting. His feelings. And he thinks that this is a brave as Will can show himself to be right now. But it says everything, doesn't it?
The ball is in Mike's court. He is pretty sure *without being certain*, that he has read the situation correctly.
So when they find El, Mike looks between them like this triangle must be resolved. But there is never any time.
And then El is in trouble, and she needs him. And Will pushes him towards confessing his love to her. He has a sudden shock of being wrong. Visibly hurt, but with no time to process hope being ripped away from him while El is dying.
He purses his lips together and tries for El anyway, knowing the love for her isn't real. Not in the way she needs it to be. So instead, he bundles up his feelings for Will, and gives these to El:
"El? (Will), I don't know if you can hear this, but... but if you can, I want you to know I'm here, okay? I'm right here. And... I love you. El (Will), do you hear me?
I love you. (But it sounds insincere because it's directed to the wrong person)
I'm sorry I don't say it (show it) more. I... It's not because I'm scared of you (Will). I'm not. I've never felt that way. Never. (Even when you were possessed. Even though you STILL have a link to Vecna). But I am scared (terrified) that one day you'll realize you don't need me anymore. (Like when you moved away from me and didn't even call. What will you do when you know about this thing I am hiding?)
And I thought if I said how I felt (I'm in love with you, Will), it would somehow make that day HURT more.. But the truth is, El (Will), I don't know how to live without you. (It's Hawkin's. It's not the same without you. *I still haven't told you I jumped off the quarry*)
I feel like my life started that day we found you in the woods. (On the swing set. And I asked. I asked if you wanted to be my friend. It was the best thing I've ever done.)
You were wearing that yellow Benny's Burgers T-shirt. And it was so big, it almost swallowed you whole. *Rewritten to frame El. And it's the only thing she responds well to, because her oversized shirt was the only truth.*
And I knew right then and there, in that moment, that I loved you. (He was drawn to, and adored Will, immediately).
And I've loved you every day since. I love you on your good days. I love you on your bad days. I love you with your powers, I love you without your powers. (I loved you before the shadow monster, and after whatever that did to you. The trauma. The possession... The connection that never quite leaves you). I love you for exactly who you are.
You're my superhero. (TF 85: Can I tell you a secret? Will is bravest person I know). And... I can't lose you. Okay? Do you hear me? I can't lose you. You can do anything. You can fly. You can move mountains. I believe that. I really do." ("In real life your powers are innate.")
And this part is just for El-
"But right now, you just have to fight. Okay? El. Do you hear me? You need to fight! You have to fight. Fight! FIGHT!"
Poor El is constantly being used to redirect or hold Mike and Will's (terrified) feelings for each other.
Even as El *dies* in s5, Mike's flashbacks of El also have Will in them. She never escapes the unspoken shadow over their relationship.