Grreaat responses.....!!!! Seriously...
My take on it is that lifelong happiness would indeed make us stagnate...
Firstly, for your information, the background of the quote is a statement regarding the theoretical use of opiate-based medications to treat psychological disorders too, not just as a physical pain-solving medication; subsequently, if they were to be used to treat, say, depression, and make those taking the medicine have lifelong happiness, would it create a stagnate quality to their lives?
And while, considering you all didnt have that background unless you read the site, you've all deviated from the topic (that is, opiate based medications causing stagnate qualities) I think that the deviation was for the better because with that topic theres not really much room to move around.
So, having gotten that out of the way,
...I believe it was either Socrates or Aristotle that said that everything in the world is generated from its opposite: fire from water, cold from heat, happiness from sadness, high from low, big from small, love from hate, and so on. Both respected philosophers of their times, the idea they provide is that one thing cannot possibly exist without the other.
In the world, if sadness did not exist, then happiness would not exist. Happiness is the term we use to describe an elated state of mind. But the term 'elated' describes nothing more than the superlative form of being merely content, in other words, not sad.
If lifelong happiness were a reality, and sadness did not exist, then 'happiness' as we know it would not exist, and by that reasoning, lifelong happiness would not exist either. Therefore, aside from it making us stagnate, it is also an impossibility.
But let's say it is a possibility, and people were happy all the time. What would be the point? Speaking from experience, and from the testimonies of others as well, when one is happy, one doesnt strive to accomplish anything whilst in a state of pure contentment. Rather, they want to enjoy it! But when one is sad or depressed, action is required to metaphorically dig themselves out of the depression.
The very defintion of 'stagnant' is
a. Showing little or no sign of activity or advancement; not developing or progressing; inactive
b. Lacking vitality or briskness; sluggish or dull: a stagnant mind.
Thus, the lack of action that occurs in purely content individuals constitutes 'showing little or no sign of ...advancement' and also represents inactivity.
And whether or not Socrates/Aristotle was correct in saying that things in this world are generated from their opposites, lifelong happiness would, in my opinion, undoubtedly create stagnancy.
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Again, great responses I'm really impressed concerning the depth with which people turned this simple question into a real discussion.
Peace...
Vaya