(I'm not certain what "should" means in that sentence - a moral judgement? - but I'll interpret it as advice.) I think that this is excellent advice for some people, but poor advice for others. I don't think there is anything wrong, per se, with wanting to be the very best at something. There's just a trade-off: you can be happily excellent, or you can take a chance and be perhaps the best but most likely just unhappy. But this decision is, in my opinion, a personal one: happiness does not have to be the ultimate goal of life.