For many anime fans, Yu Yu Hakusho was an entry point into the shounen genre or anime in general. The characters and story were some of the highest quality at the time, with arguably one of the best tournament arcs of the genre. While it started slow, the series gradually ramped things up to the point that it became a staple series for many fans. However, Yu Yu Hakusho carries yet another distinction: it was probably home to your -- and many others' -- first anime crush.
For years, sites like DeviantArt and Fanfiction.net were littered with fans of the series creating original characters to ship with one or more members of the group. Hiei and Kurama were some of the more popular choices, given that Kurama was the pretty boy and Hiei was the moody, emo gremlin he is. Yusuke was also seen frequently, as well as Yoko Kurama and Koenma in his older form. Even Kuwabara got some love every once in a while. The girls, particularly Botan, were also equally as loved by fans.
Anime crushes weren't a new concept at this time, as series like Sailor Moon and Dragon Ball Z were popular sources for husbandos and waifus, but Yu Yu Hakusho provided a more diverse cast of characters, with both male and female protagonists for fans to gawk over. While mainly aimed at young boys, the series is fondly remembered by anime fans of all genders and orientations. This was primarily due to how well the characters were written.
Though they did fit certain stereotypes, Yu Yu Hakusho's characters they were multidimensional, going beyond those tropes and actually changing as characters over the course of the series. Fans got to watch these characters learn and grow in real-time, making the connection between fan and character that much closer. It also helps that the characters were closer in age to the target demographic, as opposed to being adults.
Kurama was a loving son who struggled with his own (demonic) shortcomings while trying to find his identity and place within the Human World. Hiei was a grump who loved his sister and friends -- despite never allowing himself to show or say it -- while trying to prove he was more than what he appeared. Kuwabara was a hopeless romantic and a ride-or-die friend that would always put himself on the line for a friend, while Yusuke had a strong moral compass and would do anything to protect the ones he loved. It's easy to see why any fan would have a crush on any of these characters.
The advantage Yu Yu Hakusho also had was that there was really only one canon pairing, Keiko and Yusuke. Kuwabara and Yukina were hinted at, but nothing was ever made official in the series proper. This allowed fans to freely ship their own characters with the YYH cast without having to worry about how to get rid of pesky love interests. Most of the major characters were canonically single until the end of the series, so this made pairing easier for fans.
Even today, fan characters are created just to pair with their creator's favorite anime and manga characters. My Hero Academia is a particularly popular title to see these types of characters created for. Again, while not the first series to see this kind of fan interaction, the way Yu Yu Hakusho handled its character development surely played a major role in the ensuing shipping boom.