What makes a Protagonist

What characteristics makes a character great, especially the protagonist of a story? What makes a protagonist a protagonist? What is it about this characters that makes them great? Is the main character always the best or popular character in a story? I was very curious about this so I went online to find some facts to solve my questions. First I decided to look for what exactly a protagonist is and what the oxford dictionary says is that "the leading character or one of the major characters in a drama, movie, novel, or other fictional text." Basically the central person of a moving plot. But it didn’t answer my question about what makes them a protagonist. Is it just the story focuses on them more or is there an outline that states what a protagonist is. 

So I went online and I often found that many sources have some commonality with each other. On a site called Fiction University I found some great things about what makes a protagonist great. A protagonist must have a problem that they need to solve, an ability to act and reasons, has something to lose or gain, might have the capacity to change, has an interesting quality, flaw and or a secret. Lastly and perhaps the most important aspect is that there is someone or something that is interfering with them and without a conflict in a story the entire plot would just fall apart. This wasn’t the only site that great advice other suggested many things until I realize that all those traits have something in common. They were qualities of a hero. But not all protagonist is a hero sometimes they can be an antihero or be a flat out villain in the story. But if they all have the traits described in the Fiction University site then there is a guarantee that the character will be successful as the protagonist in a story. 

I also learned  that sometimes a story will have a character as a  protagonist, instead  they end up being killed of unexpectedly. This is called a false protagonist often used to make the plot more exciting  by fooling the audience's preconceptions, by making  a character whom the audience assumes is the protagonist but is later revealed not to be. There usually introduced in the begging in is rid off later in the story often killed off. This technique is often used in thriller and suspense and most of the time an example it was  used in the movie, Psycho. 

Either way my question about what makes a character the protagonist  has been answered but why that character was chosen as the main character has yet been answered. But my guess is that they stand out in some way like they have a special skill or have a special heritage of some kind. So what I think a protagonist is that character that has something special about them that they use to solve what ever problem is in their way. Because don’t all stories are like that somehow?