In Season 1 most people consider the fact that Saito preferred Louise before she was under the influence of the Love Potion as an Idiot Ball moment, but think about it, he actually loves her the way she is.
Again in Season 1, Saito comes back to save the day, despite having no obligation to help fight.
Season 2 episode 4, when Saito confesses to Louise and they make out for several minutes in a boat.
In Season 4, when an Ancient Dragon goes after Void mages, the place to fight it is decided to be the Academy of Magic. All students are evacuated... except for Louise's classmates — who decide to stay and fight the Eldritch Abomination alongside Louise!
Also, after Kirche learns of Tabitha's dark past and sees her haunted by nightmares in her sleep, she decides to lie beside Tabitha and embrace her, telling her everything will be alright.
Season 4 takes the cake. Saito proposes to Louise in front of everyone. Louise doesn't even try to hide her happinness and accepts it wholeheartedly. Then they get married. FOR REAL. Possibly in the same chapel where they got married in Season 2. With everyone (meaning the girls + their teachers + the royals + the elves + LOUISE'S FAMILY) attending and genuinely happy for the two lovebirds. Playing to the tune of 'I SAY YES (Wedding Version)' from Season 2's final episode. If that's not heartwarming enough, then let's go for the epilogue. Louise and Saito arrive at their newly rebuilt manor in Des Ornieres. Then they get off, Louise uses World Door, and she and Saito go to Japan. Louise experiences our world for the first time, including riding the bullet trains, sightseeing, and eating crepes. They finally reach Saito's house, where they push the doorbell hand-in-hand, their wedding rings glistening on their left hands' ring fingers and Saito's Gandalr runes still visible. Louise doesn't look annoyed, or angry, and she's really enjoying their precious time together. All this to the tune of Louise's happiest (or at least the only one where she doesn't snap at Saito) rendition of 'Kiss Shite Agenai'. All together now, D'AWWWWWWWWWWWWW~...
Meta: The author passed away with the hopes of finishing the novel series at 22 volumes. Four years after his passing, he gets his wish; the series continued to completion due to his team's hard work.