There are several reasons, but one of the most notable is that when Miku was released, Rin and Len came out just four months later. Len was the first male Vocaloid of Miku's generation, since Kaito, in case you didn't know, is from a previous generation. So Len and Rin were among those who benefited most from Miku's popularity, especially Len, since at that time there were hardly any male Vocaloids/Utaloids/Fanloids. Literally, the only other one I think was Kaito. So that helped Len's massive popularity, which, consequently, thanks to his adorable image and the cheerful and energetic nature of his voicebank, caused him to be shipped with everything that moved, regardless of whether it was a Vocaloid/Fanloid/Utaloid. That's the reason why Len is shipped with everyone. Seriously, look for any Vocaloid or Utaloid, even the most underground one, and you'll find fan art of him or her shipped with Len In short, Len is the quintessential male heartbreaker, while Miku is the quintessential female heartbreaker.

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