He left Wesr Wing because

That happens all of the time. The show is supposed to be a lead for an actor, and then the ensemble becomes the star. Or, it is supposed to be an ensemble, but one actor stands out and becomes the star.

Think of it this way. If you were told you were going to be the department head for your division, answering only to the board of directors, would you be happy if after a few weeks on the job they then told you that you had to run all of your decisions through a committee of department heads?

Apples and Oranges. Yes he was supposed to be the lead actor with the main story but it became about the President rather than about one of the staff. Sam Seaborn became the heart, weary idealist, to the administration. Rob Lowe should have sunk his ego and rode the series to the end. I think he got bad advice from his people.

"Apples and oranges" means a person is comparing two unlike things. My comparison was correct. Lowe was told one thing but then it became something else.

I will concede that I think it is stupid to walk away from a hit show where you have a character who is actually developing. But, the fact still remains that he was supposed to be the lead and it got changed to an ensemble cast.

You don't like him? Then DON'T WATCH him. He's fine, he's decent, he's f-ing HUMAN! WHAT..YOU don't make mistakes? YOU don't goof? HA!! "Give me a break...All you nasty creatures passing JUDGEMENT on this man...you should ALL..ALL OF YOU...take a very long hike, right off the very shortest cliff you can find. I'll be DELIGHTED to check a map...JUST FOR YOU!
Shut your trashy mouths, and leave him alone. (He, and COUNTLESS OTHERS...)
How DARE you.

...Rob? Is that you??

...or, maybe the viewers decide who is the star, not the actors, not the producers. He was good, but Sam Seaborn was already becoming tiresome. I think the viewers ultimately don't care who was promised what, if Martin Sheen was more compelling, then he was going to be the star. He is easily 100x the actor that RL is. I don't necessarily fault him for leaving, but the fact remains that no viewers left with him.

I don't know how old you are, but look at Happy Days
That show was supposed to be based on Richie and his Dad
But Fonzie became the breakout runaway Superstar