I main Yoshi because I love Yoshi. He is my favorite video game character, and he has been my favorite video game character since the early 1990s. I like the fact that he is cute, that he is a dinosaur, that he is always eating things, and that he is always using eggs. I also like his personality. Also, green is my favorite color, and the main Yoshi is green. Whenever he is a playable character in a game, I always choose him. (For example: choosing him in Mario Party games, Olympic games, Smash games, etc.) I just find Yoshi to be awesome in general, and I like everything about him. He is just such a lovable character.
My favorite move in Smash games is egg lay (when Yoshi puts an opponent in his mouth, swallows them, and then poops them out as an egg) -- it's my favorite move because it is Yoshi's signature move; I also like using egg lay near the edge of the platform so that the opponent falls off the stage while still inside the egg. I also like that Yoshi uses his mouth to throw opponents.
In terms of Smash games, I am a "fighter loyalist" -- that is, I will always main Yoshi in every game regardless of how many buffs or nerfs he gets, and regardless of his placement on tier lists.
Also, I like Yoshi's uniqueness -- I like that he is the only character that can use egg lay (except for a Kirby that has copied Yoshi), and that Yoshi has been the sole representative of his franchise in all 5 smash games (a trait only he and Captain Falcon have) -- though, it would be nice to get a 2nd representative. When I say "sole representative", I'm referring to the "egg symbol" that the game uses, as well as the "Yoshi's Story" theme that plays when Yoshi wins (instead of the "Super Mario" theme). Another unique aspect about Yoshi: he's the only fighter with his own unique shield (an egg).
Also: in terms of this post and other posts: are we talking about Yoshi in the individual, or Yoshi the species? I love both, but oftentimes people will say "Yoshi is [fill-in-the-blank]". Are they talking about the most well-known Yoshi (i.e. the green Yoshi that appears in most games), or are they talking about Yoshis plural (i.e. all Yoshis within the Yoshi species)? It's worth noting that a variety of different Yoshis (blue Yoshis, yellow Yoshis, black Yoshis, etc.) have different personalities and abilities.
Side note: I'm not happy with a certain meme -- a certain meme which could be considered an attack on Yoshi's image (defamation / making fun of him). I don't like the fact that whenever Yoshi is brought up in a conversation these days, the only thing people talk about is that bleeping meme (it has become far too viral). In YouTube videos about Yoshi, the majority of comments in the comments section are about this stupid meme, and not about the video itself. The meme may be an extension of Yoshi hate (and Yoshi haters).