Our married PE teacher was grooming my classmate when we were at a high school. Why do you think he did it?

So when we were at high school our PE teacher who was around 25-27yo was exposing feeling for my friend. It was really seen by all our students. He also had been our teacher since we were 12yo.
The first weird memory I keep was when he joked about «being conceived in the elevator”. In our language there is phrase “be born in an elevator” that applies to people who don’t tend to close doors but he said in his own way that I’ve never heard. We were around 13yo. At her 14 during on extra sport class he asked others not to offend her (jokingly). At her 16 he sent her on online gift on social media.

The real story started happening at high school (16-18yo). She was a sporty girl and they would go together to competitions. They talked a lot could have private conversations during classes outside. He could like her photos on social media. There was nothing particularly inappropriate but everyone saw that something was going on. They had some sort of special relationship, like they could have and argument or he could really cheer her for success in sports. Once she told me he gifted her adidas t shirt for her birthday. Once they had an argument about something competitions wide and she used swearing words and he told her that he wouldn’t allowed to other student because he somewhat loves her (he didn’t mean a romantic way but anyway).

Also we caught that he was constantly checking her account where she could get anonymous questions from students. And we caught him listening to her playlist and new songs she add on her media. This shows he also stalked her online so means he was really interested in her. Once she resent me messages where he thanks her for the cake she and her friend baked for her birthday emphasising he especially liked pineapples there.

He was married and had a young daughter. He stopping post in his wife online when my friend was around 16. Although he posted photos from championship and my friend was also in the picture. He has been working at schools for years and he didn’t post any students before or later.

My friend was a very interesting sporty girl. She wasn’t confident though and wasn’t getting romantic interest and compliments from boys. She also had problems in her family – her dad was an alcoholic.

In a few years after our graduation he posted that his love bore a son for him and that he loves her. I don’t think if my former friend and he had any contact after school but I noticed they removed each other from friends list.

So basically he didn’t do anything too much, like told her he is in love with her or do something really really inappropriate or say something sexual. I feel like he had platonic feelings for her ( even the fact that he was secretly listening to her playlist ans checking her acc where she answers questions). But it’s so dodgy given the fact she was a minor.

So what do you think it was? Maybe he was very unhappy in his marriage and found a happiness and daydreaming in his feelings. Or he was a proper predator and was trying to groom the girl into relationship?