I agree, when one becomes too competitive bad things can happen. Life will be unenjoyable. I suppose, like all things, there should be a balance to how competitive one gets.
I tend to notice birth order has an effect on how competitive and successful most people can be. I know it's not always true, but I notice the youngest siblings tend to not be very successful. Babies of the family tend to not have to compete for much. This transfers into their adult life and usually professions. I've always noticed, I many people I dislike are the youngest. While many people I get along with, and share similar goals with, are oldest. However, I have to say, many only can be goal orientated and competitive. It seems to be a split. Onlys either take on the personality of a "baby" or of the oldest because oddly they're both.
I think even very competitve people have one thing they strive at. It's human nature to want to be good at something. I know there's certain things I'm more competitve about.
I think Dr Kevin Leman's theories are interesting, but they are also generalities. to me, much of his teaching tends to seem very much based on his own experience. I am an oldest, & I'm not competitive. I push myself to do things, but I march to the beat of my own drummer & I'm not concerned with what others are doing.