It is an old saying attributed to Buddha by some and to Budai (Laughing Buddha) by some.
It doesn't matter because we are only going to look at its import.
The import is, though pain is inevitable, suffering is not.
Implying, one can chose not to suffer.
Now the question is,
Is it possible to chose not to suffer when we are in pain?
To answer that question we will have see what we mean by suffering.
From our experience we can see that suffering is not about sensation of pain but mentally enduring the pain.
So, pain may be physical or psychological, but suffering the pain is only mental/psychological.
Which means suffering is a state of mind.
If it is state of mind then it is left to us to decide how we suffer the pain mentally.
We can see the difference more clearly by analyzing a very simple situation.
For anyone to walk a long distance under hot sun would be painful in a way and state of mind would be that of suffering the heat.
But, if there is substantial reward at the end of the walk, then state of mind would undergo change.
Now, walking under the hot sun won't be perceived as suffering under the heat but only as a difficulty.
The state of mind changes entirely, now the mind is ready to accept the pain but doesn't suffer it mentally.
That is, the mind chooses not to suffer and so names it as difficulty.
Though quantum of pain remains the same but perception of suffering undergoes a change.
This clearly shows that pain is a fact and suffering is only a state of mind.
So the saying 'pain is inevitable; suffering is optional' is a factual statement.
The situation stated above may be too simple.
But, one can extrapolate it to more difficult issues involving physical or psychological pain and still find logically, suffering is only a state of mind.
Of course, it may not be easy and not even possible to practice it under all situations and end mental suffering.
But, people like Buddha/Budai always talks in terms of absolute, one can't derive relief or relative comfort from them.
So, at the least this is the direction one has take to gain insight into suffering and understand the anatomy of suffering.
We would be able free ourselves from suffering to the extent of insight and understanding we have about it.
