That's true, but rather than "make-up" they are assigning a reason to their actions because they did them voluntarily. That is a rational thing to do, that gives a why for their action through their introspection, or sheer need for justification. If it is not voluntary, they'll often say that or it is immediately apparent, or they are ill.

And we are fully capable of investigating that reason, that why, to find out whether it is reasonable or whether a better why can be provided. Animals cannot assign a reason, nor can we "investigate an animal's reason", because that's nonsense. Animals don't give why's.