It's not for selfish reasons it's stupid and was stupid how they did it. If they would have set it up better I would have mind, but prior to the confession they had no chemistry at all. None. All they did was argue.
1-Tatsumi himself was confused by the confession:
He was so confused because it was stupid. He even said himself that any team mate would have picked her up which common sense. If you fall and someone helps you up do you love them? No, you like them a bit more.
[question]2-Which is why he thought she finally accepted him as a friend
Because why? They weren't friends before, all the time they spent together was arguing about why how did couldn't stand the other. When they went shopping, they argued about hey they were wasting time and how Tatsumi wanted to leave. On the cliff with the prophet they argued about how they didn't like each other where he confirmed her death flag.