Isaac doesn't exactly mind when Scott's hand tightens, it followed through with the sense of the movie at least. Its what happens next that he doesn't exactly know how to deal with, he felt like he was intruding almost because clearly Allison's hand being so close to Scott was the catalyst for this.

He laughs out of nowhere, making the whole situation awkward but everything was bordering on the edge of hysteria. He looks at Scott for a moment before turning back to the movie, trying to forget about the whole thing because if Isaac himself paid too much attention then similar things might happen to him and that would be a lot worse considering Scott's hand's proximity.