It's a reason why we care so much about this two characters. We see them grow. The thing is we don't see the characters falling in love, we see them choosing love. We know there is love between them, even before the story starts. It was always there. But we see them to realize that they're in love. We see them accepting the love for the other and believing they deserve it.
Friendship is not less than love, it's love. Because it's possible the existince of ways of love like different kinds of people. You cannot measure it, you cannot tag it, but you can choose if it's grows or dies.
Colin and Penelope love story was destined to grow. Because that was their choice.
The most important aspect of their relationship it was always there. They inspire each other. They always cared for the other. And this is the reason. Because we can talk about first crushes, who knew it first, or why they love each other. The important thing, it's what this relationship creates between them.
They believe in the other. They always did. The bond between them makes them want to be a better person. So the line between a frienshship and love starts to disappear. Because romance it's about connection. To be close to the other person, and choose the other. It's a decision to give your life to be with this person. Pen sees in Colin someone kind, she valued this very much. It's possible this the seed that make her starts to love him in the beginning. She tells him this in 3x02 and 3x07. She's romantic from nature, so to her it was more easy to connect the dots. The need of connection she has with him makes her want to have a more intimate relationship, because she wanted more, specially kindnes. The lack of love in her life make her appreciate more the love she receives. But she doesn't feel worthy of it. She doesn't have the self-confidence to go for what she wants because she was teached she couldn't have it.
Colin on the contrary, it's more blurry. His mouth says one thing but his actions do something else. He doesn't see what is in front of him, but that doesn't mean he cannot feel it.
Everything decision he makes make him more close to the same result: Pen.
He's always choosing her.
Because maybe he didn't understood why or what it was but Colin always knew something: Pen makes him happy. It's sound cheesy, but it's not. She creates a feeling in him that nobody else does. That's why he is always seeking her out. When she says to him, he's kind and sweet. He says she's warm and clever. That he feels seen when he's with her. She makes him feel that he has value. Specially the parts of him that nobody else cares or sometimes even dismiss.
You can say that he loves her because the way she makes him feel, but I don't think it's the case. She makes him feel that way, because he values what she's.
He sees her good nature and that is the reason why he's always looking for her. She's not an idea in his mind. She transforms. She forces him to move, to be, to evolve.
She takes his breath away. She creates on him the need to be soft. He values her opinion more than anyone else. She's the person he is always looking for in the crowd.
He always acts like he has a string in his body that move him close to Pen.
But it's something more, his chest gets bigger and high when she's close. All his posture changes. Even, when he could denied it, sometimes even unconscious, he's trying to flirt and maker her seeing him like a man. He's always to close to her and he's always trying to touch her without realising.
Everytime she says something to him about it he ignores it. Because to him touch is something natural, it's about connection.
His heart gets warm when she's around. He sees her like someone sweet and kind too. That makes him want to be the same even more. The way she see him makes him believe in himself, he wants to be the person she sees on him. She believes in him, that's love, and that love gets reciprocate. Because he's not indifferent to it, he appreciates it, and with it he appreciates her too.
The love between them became something physically when the need of connection it's more strong. The letters made a romantic connection between them, specially because they create the illusion of exclusivity. The letters are something that is just between them. Nobody else can read them, nobody else is looking through it, not even Eloise. It just the two of them. It's the beginning of an intimate relationship. The connection was there before, and that's the reason because the letters exist. But they were a safe place for both of them being themselves and open to each other. So, when the letters stopped, Colin felt alone, and his need of connection grow so desperately that he tried to reach it how he could. But nothing was feeling the void. He missed his relationship with Pen and he wanted that relationship back. He knows for the first time what it feels not having her in his life, he doesn't want that. He hated it. He felt loss.
So, when Pen choose let go of him because she wants more from life, even when she still loves him. She knows she would choose him without even thinking about it, but Pen doesn't feel that she has that option: love is not an option anymore. In that moment he's choosing her, even more than before. So, he push back. But then, their relationship was never going to be the same.
Their friendship becames something that can't include other people, even when they tried. The need of connection it's more intense that it was before. The sexual attraction is too, for both of them. Because the need for the other is more stronger. So, the idea of one of each other being with someone else, physically or emotionally, becames impossible. Colin starts to understand what was the root of his need for her. He doesn't know if she reciprocates this, but he needs to. Because he does something that Pen did before him: have faith. She loved him even when she thought he didn't loved her, because she had faith in her love. She wanted love, even in the door to marry someone else, someone who wasn't offering love to her. She stills was waiting for him to go to her, she believed her love for him was enough. When Colin goes to the ball to search for her, he does it because he has faith in her but specially in his love too. Even when he didn't know he was going to be reciprocated, he chose to love her without the guarantee of being loved. He's choosing to love, and that was a courage move. Because love is a choice that it requires courage. And they chose each other again and again without thinking about it, every time.