The problem is, that, say, the ability to laugh, while definitely present in all beings that we agree to be human beings (on other grounds), and definitely absent in all other beings that we agree (on the same grounds) to be not human, is nevertheless considered to be accidental with respect to human nature.
However, it is not unreasonable that this (empirically ascertained) universal and at the same time exclusive occurrence of this property is at least an indication that it necessarily proceeds from human nature, and thus, that it is not accidental.
The alledged accidental nature of the event that human nature 'meets' ability to laugh in all observed cases is hard to believe, but cannot be disproved because the evidence in favor to its being necessary is only empirical, it can be dismissed as evidence as soon as some observation is made of a case where the two have not 'met' (for instance a being, which we all agree to be a healthy and undamaged human being, which, as it turns out, has a (local) morphology and physiology such that it cannot perform laughing). Only then we know that the ability to laugh does not necessarily come from human nature (assuming the latter has been correctly defined).