Well, first of all, Richard Spencer didn't "start" anything. He just owned a site called altright.com, and that "alt-right" brand took off, labelling a bunch of people from different sources. As it happens, altright.com was like the least popular of them, lol.
As for his own personal motivations, I think a lot can be gleaned from his spat with Nicholas Fuentes. I remember Richard saying that Fuentes would've had a much easier time at life if he just became another generic GOP party hack. This was clearly projection. Spencer was talking about himself there. He could've become a generic GOP party hack if not for his own narcissism and need for attention, which he extracts through outrage
It was clearly projection because at that point Richard was divorced, bankrupted, and ruined, while Fuentes was doing quite good for himself. Also he was clearly jealous at how much more popular Fuentes was than him, and within his "own" movement!