HDDCS was a reboot of Woh 7 Din, which was also the remake of a Tamil film, which was based on a real life story.
Manmarziyaan made by Anurag Kashyap again was a reboot. The original film shows the girl end up with her husband instead of her lover, and all the remakes follow it instead of changing the ending no matter in which era they were made.
Because the core message of all the films is that the "love" experienced by youngsters is nothing but infatuation, and parents would always choose a better guy for marriage than you.
Bollywood has sold the idea of young innocent love as the greatest power in the universe in so many films, that our generation is never going to like a girl ending up with a partner who was forced by their parents.
Anurag Kashyap knew this and that's why Manmarmaziyaan didn't give a shit about the sanctity of marriage, instead tried to explain why Vicky Kaushal's character who is the lover, is not the right guy for marriage because of his irresponsible nature. While Abhishek Bachchan playing the husband was a mature guy.
Vicky and Tapsee got many chances to unite and be together, but their "love" was just not strong enough, because it wasn't love but just infatuation.
HDDCS completely fails in this department because SLB as usual was more interested in showing his grand sets and hero-heroine dancing. Vanraj might be a better guy than Sameer for marriage according to many people, but love is not always about who is "better".
More than half of the movie is about Nandini going crazy for Sameer, standing against her family, trying to commit suicide, even convincing her husband to take her to Italy, and in the end rejecting Sameer because you're married and you got a "better" guy?
That definitely won't fly well with many people because like I said before, love is not all about who is "better". After showing Nandini do all the nautanki for Sameer, trying to convince the audience that it was all just an infatuation ain't going to work. It's just bad writing. SLB wanted to show melodrama because he loves it.
When Woh 7 Din was released, it was easy to convince the audience on why the woman should end up with her husband and not her former lover, because the alternative was unthinkable for them.
Manmarziyaan released at a time when the idea of woman leaving her husband to go back to her lover wasn't new, so director got into the characters psyche to convince us on why the the woman should end up with her husband.
HDDCS which released in the middle was released in the era when SRK was fooling the audience by showing young innocent love as the greatest thing in his films. This was obviously going to make the ending of HDDCS controversial. On top of that, SLB was just not capable enough to write a mature story.
The end result is people debating the ending of this film till this date.
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