Under all of that hatred and evil that accompanied Draco wherever he went there was always something pure and sweet that was outlining his true heart of gold. The metaphorical mask that he used to cover up his true self, his true feelings. It didn't just happen overnight his feelings for her just crept up on him slowly.
It was in third year when he started to feel different towards her. After he called her names, horrible names, he felt horrible about it like he was disgusted in himself for making her feel miniscule, like she was nothing to him; even though deep down she was his everything.
He is mow in his fifth year and he has just embarked the Hogwarts Express. For the first time since school ended he saw her. She was so beautiful she literally took his breath away every single time he laid his eyes on her. He felt as if there was no one else on this planet but her and himself.
He would have told her about his feelings towards her but he didn't for three reasons. One, he couldn't find to will and the power inside of him to do so. Two, he didn't deserve her she deserves someone who treats her right and will always be there for her through thick and thin. Three, his parents would disown him; she is a muggle-born witch and he is a pure-blood.
However this time it was different, instead of having a school-boy crush on her it turns into something more powerful; in fact it is the most powerful thing on this planet.
Love.