Yu-Gi-Oh 5D's gave a good reason why power of friendship works in-universe

Well, it has a good reason to exist in any series since cards in Yu-Gi-Oh are actual living beings that will respond to your belief in them and allow you to top deck. But 5D's, specifically the second half, gave another good reason.

FYI, Yu-Gi-Oh in general is weird, so bear with me.

One of the main points of 5D's is Momentum, a self sustaining device that is cause of limitless energy. 5D's protagonist is Yusei Fudo and his father created this device before he was born. 17 years before the series began Momentum malfunctioned, splitting Domino city into two and killing thousands in an event known as Zero Reverse. This was revealed to be work of time travelers that wanted to prevent a much bigger Zero Reverse from happening and killing all humanity in their timeline.

As it turns out, Synchro summoning, main gimmick of 5D's, caused Momentum to spin rapidly in the future, which created Mechlords - which are essentially giant terminators that started killing people that had negative emotions. When they realized humanity is doomed, they self destructed with Momentum in a sucessful world wide kamikaze attack and killed everyone but 4 guys.

We are introduced to a new character called Antinomy who tries to teach Yusei a new form to Synchro Summon called Accel Synchro. Antinomy tells Yusei that Momentum that powers their duel disks/motorcycles can read a persons heart and that he needs to have clear mind in order to perform Accel Synchro and find a way to stop Mechlords and prevent a new Zero Reverse.

Yusei believed in power of friendship. He grew up in Satellite, a junkyard island on coast of Neo Domino City and had to struggled in order to survive since Satellite residents were seen as scum by people from the city. Despite his friends knowing his dad created Momentum and was reason for Zero Reverse and their poor upbringing, they never held any grudge towards him. Yusei believed himself unworthy of their friendship, suffered from deep survivor guilt and despite preeching about bonds and friendship at first, often times tried to push his friends away and go at it alone because he wanted to sacrifice himself to atone for his father's sins.

Fun fact: This is why [Stardust Dragon](https://www.google.com/search?client=ms-android-samsung-ss&hs=Mt7U&sca\_esv=06de5ba07996f893&sxsrf=ANbL-n6H4UKrvmN\_DnjzarUjzrJz4A3iIw:1777402302373&udm=2&fbs=ADc\_l-aN0CWEZBOHjofHoaMMDiKpaEWjvZ2Py1XXV8d8KvlI3vWUtYx0DZdicpfE1faGYenqWn-q4MFiFFtvJjTKeAVxBf9XF8ByrMpEedseJb6C24e7QdJQdIE3TPpl5mEwf0EBCRLNeEhQy5amsEYIcoTye3rrZrd3IP3OYha6\_rH\_GIVOU8GK5eecabclKqwVhxmAmgM5&q=stardust+dragon&sa=X&ved=2ahUKEwj1j-zGm5GUAxW5GhAIHWdaLycQtKgLegQIEhAB&biw=384&bih=701&dpr=2.81#sv=CAMSVxoyKhBlLUNianI5dmt5MWJMUHpNMg5DYmpyOXZreTFiTFB6TToOdE1TUnJOSTdWREdpWU0gBCoXCgFzEhBlLUNianI5dmt5MWJMUHpNGAEwAUoECAEQAhgHIK7ny7gMSggQAhgBIAIoAQ), his ace card, has an effect that protects other cards from destruction - symbolizing Yusei's self sacrificing nature. His secondary ace, [Junk Warrior](https://www.reddit.com/r/yugioh/comments/1omu5gy/thoughts\_on\_junk\_warrior/#lightbox), has an effect that increases his attack by all level 2 or lower monster - symbolizing Yusei's belief in bonds and that nobody is useless.

In his duel against Placido, Yusei finally let got of his fears and insecurities, cleared his mind and managed to [Accel Synchro summon Shooting Star Dragon](https://youtu.be/clM4bjzGMGI?is=vFXTz5CDZJ-KPjUr) in what has to be one of the best moments in all of Yu-Gi-Oh.

Except that merely using Accel Synchro isn't enough. After all, Antinomy taught him that and he came from a future where, despite knowing it, it wasn't enough to fend off Mechlords.

In final duel against the main antagonist, Yusei is given ace cards of all of his friends and duels the Z-One - the last survivor of the future timeline. Yusei was pushed into a corner, but because he believed in his friends (and had dragon deity on his side), he flew into space with his motorcycle and used all dragon cards his friends gave him to go beyond. So he [Limit Over Accel Synchro summoned Shooting Quasar Dragon](https://youtu.be/AIj3q7jQIp4?is=yqLXOjGRc3vVCi4x) which allowed him to defeat Z-One and save the future.

Yusei believed in bonds with his friends. And it had a lot of narrative weight backing it.

Anyway, Yu-Gi-Oh 5D's is peak, so you really should watch it.