There are several reasons for this: the main reason is the sound and the style. Cowboy Bebop was among the first series I watched, and Trigun shares many of the characteristics that made Cowboy Bebop so great. Here, various stereotypical Western themes are used and the music keeps the series going, adding the necessary emotion to every scene.
Moreover, let me just say, Trigun has the best openings I have ever seen. The gargantuan guitar riff driving the OP would leave you to believe the series is just filled with action-packed gunfights, with an incredibly cool protagonist.
In this respect, Trigun is deceptive. Vash the Stampede is a cool character, yet for nearly every awesome action he performs, there's an equally ridiculous and idiotic gesticulation to counter it. While it certainly creates a memorable protagonist, it can lead to some frustration as it means his personality can never be pinned down and defined, due to this constant subversion.
I do not mind this one bit. The first half of the series is mainly a series of comedic adventures, which suddenly turns into an altogether different beast halfway through the series. The darker theme that explains the series’ main story arc still contains comedic moments, but these are dropped in favour of narrative, and the ability that Trigun has, to juggle comedy with bleakness, is outstanding. However, the main draw of the series is undeniably Vash, and if you do not find yourself entertained by such an incredible character as his, it is doubtful you will enjoy the series. Fortunately, it is nigh on impossible not to appreciate such a likeable character as his, which makes such an amusing and entertaining series.
