Ahh I get it now. He's not looking at the girl in the black top, he's looking at the empty seat at the bar. Then at the end of the MV there's a girl in a red dress sitting in that seat. From the way he looks at her legs, i'd say he's really into her. And seems like she's his ex, because how else would he know she was supposed to be in that seat at the start of the MV. He probably picked the restaurant on purpose, and is trying to make the ex jealous by proposing to his new gf in front of her.
One thing that gets me though is that he looks genuinely in love with the girl in the blue top at the start of the MV when he's staring at her. Maybe he's fine with her until he sees his ex, then realizes that he really can't love the girl in the blue top.
Also, this is just my personal opinion, but when he was stroking the girl's hair, it felt like he was imagining it was the other girl's hair whilst looking at her...mhm messed up