Sometimes what appears to be a serious problem is not one.
1. Why does Ariel from The Little Mermaid give up everything she has for a man she has just met?
The simplest answer is that she fell in love with him. But there seems to be more to it than a simple love sacrifice. Even before Ariel met Eric, she had dreamed of exploring the human world. But she never had even the slightest thought of going to Ursula the witch, selling her voice in exchange for feet, and leaving her family forever. But once her father destroyed in a fit of rage the collection of human trinkets Ariel had so lovingly put together, that was the final stroke for the little mermaid. She felt that her father ruined her entire life's work. She realized she could never be happy confined in the ocean, so she put everything at stake to discover a new world. So for a free spirit like Ariel, being with Eric was just a nice bonus.
2. Why did NASA train oilmen to be astronauts in Armageddon when they could have just trained astronauts to drill oil?
That's perfectly explicable. NASA and other space agencies do it all the time: when they need someone on a mission with specific professional expertise, it's easier to prepare them for the operation than to teach astronauts to perform the professional part. For the record, drilling oil wells, or any kind of deep geological work for that matter, is an extremely laborious business. In Armageddon, the drilling team consisted of real specialists in their field who were trusted with a task no one else was able to perform.
3. In Titanic, why didn't Jack and Rose climb on the life-saving wooden door together if there was enough space for the both of them?
This question has been asked by millions ever since the premiere. In fact, if you take a closer look at the scene, you'll notice Jack and Rose could not fit in there together. Jack tried to climb onto it, but under his weight, the door would have tipped over. He had to sacrifice himself to get Rose saved.
4. Why didn't the characters in Harry Potter use a Time-Turner to go back in time and kill young Voldemort?
There appeared a widespread belief after the release of Harry Potter and the Prisoner of Azkaban that if you can time travel, you can just as easily go back in time and solve all the problems, for example killing Voldemort in his youth. But in fact, the characters in the movie couldn't really change the past. When they went back in time, they actually relived the same situation, only from a different perspective, and they couldn't influence the course of events in a drastic way. So it was not an option for the characters!
5. Why couldn't the Lord of the Rings trilogy's Frodo and Sam fly eagles straight to Mordor?
Well, in all likelihood the Eye of Sauron could see everything in the sky miles and miles ahead, and the eagles simply didn't have a chance to pass unnoticed. The orc armies would have shot them down somewhere on the outskirts of Mordor, if not before.
6. In Star Wars: The Force Awakens why was Kylo Ren, trained with a lightsaber for years, so easily defeated by Rey who only held the lightsaber for a few minutes?
The obvious explanation is he was too exhausted. Kylo had killed his father 10 minutes earlier, had been shot in the leg by a bowcaster with a power of a car, and had been hit with a lightsaber by Finn. On top of that, he didn't even try to kill Rey but, instead, to recruit her. Nevertheless, despite all these obvious facts, fans of the franchise insist that either Kylo is not strong enough, or Rey received special powers from unknown sources.
7. Why did the couple from A Quiet Place decide to have another child in the middle of what is essentially the no-noise apocalypse?
It was most likely an unplanned pregnancy. In a dystopian world where there is chaos all around and the heroes have to raid an abandoned pharmacy to find leftover medication, it sounds improbable that they are looking for birth control and contraception. In any case, both of them have bigger issues to care about.
8. In Avengers: Infinity War, why didn't Thanos simply create more resources instead of killing half of everything living on every planet?
He has been toying with the idea of genocide for a long time, it's just that no one took him seriously and thought he was just out of his mind. So he probably wanted to annihilate half of the population just to say “I warned you."
9. How come in Back to the Future, Marty's parents don't realize they've met their son before his birth/didn't remember they met his exact copy?
This one’s easy: they had only known each other for a week, and it all had happened 20 years prior. Lorraine probably forgot very soon the silly teenage crush she had just before she met her future husband.
10. How did Batman return to Gotham in The Dark Knight Rises if the city was under Bane's control?
It’s a known fact that Bruce knows the city inside out, and even if the main roads are taken by Bane, he could still find his way in. The really curious question, though, is how he managed to recover from his broken spine and prison surgery to go out and fight at all.
11. Why didn't Vader sense that Leia was his daughter in Star Wars: A New Hope?
Despite all his power, he's not a psychic. He didn't sense anything about Luke either, and he was pretty sure Padmé had a miscarriage. Vader only discovers the truth about Luke thanks to Palpatine.