Maybe, I don't understand what "being mature" means to other people. Or I have another way of looking at that sentence. Because from what I read some comments, many think that Yuri is more mature than Masashi and Yamashita, so they like Harumi's decision to choose Yuri.
However, I think Yuri is the most immature of all. It could be seen from his jealous attitude towards Masashi on their first meeting that even his relationship with Harumi as teacher and student had only just begun.
Besides, when he was jealous of Yamashita, he decided not to go to school just because he didn't want to meet his teacher.
From the reason he decided to take private lessons because Harumi was also a ridiculous reason.
At first, He also always refuses help from Masashi or Yamashita only he thinks that he can be independent. (Aahh that's the attitude childish who feels himself strong without anyone's help)
Maybe many thought he was mature, when he congratulated Masashi on his marriage to Harumi which was a misunderstanding. I think Yuri also admits that Masashi is the best for Harumi because of his figure who is always there for Harumi, and his established makes him think he will always be able to make Harumi happy.
Yes, I admit that's when he became mature.

But I think here the most mature is Masashi. He has even been beside Harumi at all times for 20 years without Harumi realizing his feelings. Even though he tried to express it several times, Harumi always failed to understand. For the sake of the relationship not being awkward because after all they are relatives, he can refrain himself from acting as a cousin or brother who is always there for her.
Even he knows Yuri's feelings for Harumi, but he sincerely helps Yuri to pass the Todai entrance exam. We could see how happy he was when he found out that Yuri had successfully passed into Todai.
I'm sure people thought he was immature when he hit Yamashita because he claimed to have spent the night with Harumi. (Hahaha..Yamashita is the one who acts like a child. Why did he say that to him, I'm sure he knows Masashi likes Harumi, that's why he said that to make him jealous. Uhhhh that's disgusting)
Yamashita attitude when asking Harumi on a date also tends to be pushy. He had just submitted the divorce papers, but he had already decided to start a new relationship with Harumi. (Oooh that's ridiculous)
But I admit, his decision to side with his father-in-law to help Yuri's father was mature. That's when I saw him as a teacher figure for Yuri.

At the beginning of the story, I thought that Miwa liked Masashi, it was seen when Masashi asked him to support his relationship with Harumi, but I saw that Miwa seemed to be blocking their relationship. It made me think that she liked Masashi. (Hahaha I was wrong)
But if the storyline was like that it would probably be a better ending for Masashi. Yes, maybe when Masashi decides to move on from Harumi, he can open up his feelings to Miwa, because they sometimes have the same way of thinking. I see good chemistry in both of them. Hahaha

For the whole storyline of this drama I like it.
It's just that the ending of the story is a bit forced in my opinion, because from the start I didn't see Harumi having romantic feelings for Yuri, but why at the end of the episode it was told that she actually liked him.
The author describes Harumi as a woman who is inexperienced in love. She sincerely helps a student who has a dream to pass the Todai entrance exam, she put all her might to help him by placing herself as a teacher instead of a woman who supports her boyfriend. (Yes, I saw her sincerity only as a teacher not a woman)
It would be better for me if at the end of the story she also rejected Yuri as her partner, because her feelings for Yuri were nothing more than her feelings for a teacher to a student. It's the same with her feelings for Masashi who is nothing more than a cousin.
Instead I doubted her feelings for Yamashita, her awkward feelings when she knew Yamashita was someone who was married but still comfortably drank with him. Even when Yamashita asked her out she didn't refuse. (Is it true that Harumi doesn't have romantic feelings for Yamashita? What does the awkward feeling mean? Doesn't someone who likes each other feel awkward before starting a relationship? I don't know, maybe it's just a personal question)
But still I prefer the ending of the story does not end with a romance story, it would be better if the four of them remained on good terms without any romantic relationship between them.
3 men who were rejected by the same 1 woman, but these three men are always there anytime, anywhere when this woman needs help.
(Classic ending from other Japanese dramas that I used to watch. Hahaha, maybe because I'm used to endings like that, when I was given a definite ending, I wasn't ready to accept it)

Overall, this drama get point 8/10 for me.
(Sorry my english so bad)