Looking back at my own childhood crushes, I can see how shallow and volatile they all were. I also remember that if you asked me about it at the time, I would passionately argue that what I'm feeling is true love, lash out at any suggestion that it's just infatuation that will pass and feel down for days whenever I felt something went wrong.
Totally different from my current love for Nami how I would handle such feelings today.
From our perspective, the MC's relationship with Summer might seem like a pleasant distraction, a fling. To him, it was the real deal, especially since she was the first person he had built such a connection with. Given enough time, it either would have lasted and matured or it would have fizzled out and died with a pathetic whimper.
But neither of those things happened - instead, the relationship was abruptly cut short in a way that's still shrouded in mystery.
Remember, at this point it's the uncertainty over what happened to her and the lack of closure that torment him. He's not expecting to see her again. He is open to seeing other girls but is still determined to get to the bottom of what happened to Summer so that he can really close that chapter of his life and leave it behind.