I think that this perspective in itself is wrong - because if one were to measure life-based upon pleasure and suffering, it would be pretty easy to fall for persuing suffering fundamentally. I think that the root of living intact with the good implies that you always see the higher value of things rather than focusing on potential conditions that you may approach. Life is good when one finds sense in following ethical and moral guidelines instead of living to find pleasure - it is in some way to change the thought of what constitutes a good life by implying that logic and moral values stay higher than immediate pleasure.

Firstly, it is easily understandable that people rather would fall for the idea of striving towards pleasure than higher values: this is also where the problem occurs when reasoning it trough. There isn't really a reason to choose responsibility for suffering - but if responsibility is preferred in this matter, you're good.
Hedonism comes with a cost - guilt. To act upon reason is to act upon the categorical imperative of Kant - meaning that you then have the obligation to act as you wish others would act. Hedonism is pure egoism, you do not wish a society full of people that choose pleasure over maintaining order. If we were to sacrifice everything to make the most pleasurable life, we would end up losing everything and have no real core-value to our way of life (also a reason to why many are sceptical to modernisation.)

To predicate the concept of a human nature upon animal universalities is ridiculous - the question is better put upon by asking: what distinquishes humans from all other animals? I think that we can come to a somewhat sensible conclusion by seperating humans and animals as one entity.
Sartre says that the fact that humans are put upon when living to define their nature is to understand that there are no external nature to which we can base our lives upon - and that we only can have a nature as creatures if we define the defining of things itself as a presupposition of what human nature is all about. I agree with Sartre here. We are the only animals which have to base ourselves upon a partnership (with a distance) between our external and internal self - it is important to point out that the existence of both aspects is an indicator of a human nature having possibility to exist.