When did Patrick started having real feelings for Kat ?

I'm guessing when they went on the paintball date.

Once upon a time there was a magical place where it never rained. The end.

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Definitely way before the paintball thing. My guess is it was somewhere at the party where she danced and had drinks and finally threw up on him. He didn't kiss her in the car out of respect for her, but she was pissed, so he apologized.. But he's definitely hesitant, because all he had to do was take her out, but I think he really wanted to kiss her, except he didn't want to take advantage of her in that drunken state.

When he's at the bookstore looking for the "Feminine Mystique", he's already quite into her. It's one of my favourite scenes. The one in which they walk on the opposite sides of the bookshelf and meet at the end.

When she crashed into donners car, that's when it started I reckon.
But J think he might have liked her already-or at least have noticed her.
Then there was the bit where he saw her dancing... Defintley before the paint balling bit, I reckon.