Why is immediate pleasure considered by the majority as irrational and as a trait restricted only to non human animals?
What's more rational than immediate pleasure if death is inevitable and more importantly unpredictable: it can come at any time.
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Immediate pleasure isn't strictly irrational. Rationality can usually only be understood relative to some objective function. However I'll provide two points that may assist in understanding why people often consider it to be irrational.
(1) Often one can gain greater pleasure in the future (or avoid future pain), by sacrificing immediate pleasure. For example, you may be given the choice of having one jellybean today, or two jellybeans tomorrow instead. In this case, provided you can survive until tomorrow, this may be a "rational" option, depending on your objective function. Another example is that taking drugs may be considered irrational because it often leads to negative experiences in the long term.
(2) Pleasure may not be the only component of "Eudomonia". Many individuals would for instance rather understand a terrible truth than live in ignorant bliss. The root cause of this desire may again be related to point (1), whereby not knowing the truth may expose you to future pain.
We all know about the use of fishing hook. And it is used only by humans. I am saying about this just for reminding you of cheating by showing or giving a bait. It is some people's habit to cheat others for gaining something. Certainly, some animals also cheat; but only for their food. So it is very important to know the danger behind every action and act accordingly. Otherwise you would be cheated. But humans, since they have five powerful sense organs, it is very easy to get trapped if they don't use their intelligence/power of discrimination. We need not use this (our power of discrimination) much if we consider immediate pleasure only. So majority of people consider immediate pleasure irrational. All the developments we see now are because humans most often abstain from this type of things and activities. You may think over it. And this has great significance especially if one's aim is finding out the truth of something/truth realization. Even though you have no such aims you may think how all humans would behave if all are pursuing immediate pleasure only.
Though useful, sense organs are sometimes deceptive. See this quote taken from the link given below:
In the Vivekachudamani (76) Sri Shankaracharya describes the plight of those whose senses are unrestrained: “Attachment to any of the five senses is enough to cause death, as happens with the deer, the elephant, the moth, the fish, and the bee. What then to speak of man, who is attached to all the five!” A deer meets with death when, unmindful of the danger lurking behind, it stands transfixed listening to sweet music. A mighty elephant wandering at will in the forest gets caught when tempted with the touch of a female elephant. A moth is attracted to a bright flame and rushes into it to burn and perish. A fish succumbs to taste by swallowing a piece of flesh used as bait on a hook. Attracted by the fragrance, a bee enters a lotus and gets trapped in it when the flower closes its petals at sunset. How then to describe the plight of man who, enslaved by all the five senses, loses the power to discriminate and is unable to strive for the spiritual goal.
Got the answer to your last question also (From the link)?