I picked Marth because when I was deciding on a character to main (just found about the competitive scene) I saw some matches of Mikeneko at Apex. Of course I was blown away by basically everything, but somehow the style Mikeneko used to play Marth just made me real interested in him. I started looking for more info on him and eventually discovered the flashy Europeans Marths, Mr R and Leon (and sure Leon is Canadian now, sort of) and found it interesting how differently all of these players played Marth.
Overall the playstyle of Marth seemed to be quite elegant, about precision, pretty versatile on top of which you could actually do some nice combos (Mr R's combo video the Messiah was amazing to see at the time and still just blows me away every single time I rewatch it). So I liked the playstyle, and decided to give Marth a go. I wanted to play like the best Marfs out there. Thus I wanted to beat people but at the same time play in a way I would find personally find entertaining to watch.
So these days I have mained Marth for like a year, and the more I've learned about him the more I've felt like patting myself on the back for making a good choice. He is fast, he has speed and range in his moves, all the moves he has (besides the useless Bthrow) are viable moves and make it possible to be creative, go for some fun mix-ups. He has the tools to beat anyone in the game, but at the same time he requires precision and finesse. You need to play smart, choose the correct option and at the time same time keep your spacing good. And if you manage to play smart, you will be rewarded greatly in the form of ever so sweet to land Fsmash tipper and generally being pretty safe. And since Marth does not have any BS (MK's Dsmash, UpB, Tornado, auto frametraps more or less impossible to gimp, ICs chaingrab, Diddy's bananas, Olimar and his camping and stupid Usmash, Snake and his ridiculous weight and tilts and Falco and his lasers) so you generally need to simply outplay your opponent not being able to rely on stuff like that. This can be seen on Marth's MU chart as well with him not destroying that many characters.
Maybe I am biased with my ever so high opinion of Marth being a nub and all, (yeah gets juggled superhard, easily edgehogged/gimped, not best on the ledge, RCO lag yay) but that's why I play Marth anyhow.
tl;dr Mars la Marf is awesome