Why is a man condemned to death not fortunate
If he is released after having his hand cut off?
Why am I who am experiencing human misery not
If by that I am spared from (the agonies of) hell?
If I am unable to endure
Even the mere sufferings of the present,
Then why do I not restrain myself from being angry,
Which will be the source of hellish misery?
In these two verses [from The Way of the Bodhisattva], Shantideva explains that by not being angry and developing hatred in response to harm caused by others, what one is gaining is protection from potential undesirable consequences that might otherwise come about. Because if one responds to such situations with anger and hatred, not only does it not protect one from the injury that has already been done, but on top of that one creates an additional cause for one’s own suffering in the future. However, if one responds without anger and hatred and develops patience and tolerance, then although one many face temporary discomfort or injury, that temporary suffering will protect one from potentially dangerous consequences in the future. If this is the case, then by sacrificing small things, by putting up with small problems or hardships, one will be able to forgo experiences of much greater suffering in the future.
An example Shantideva uses here is that if a convicted prisoner can save his life by sacrificing his arm as a punishment, wouldn’t that person feel grateful for that opportunity? By accepting the pain and suffering of having his arm cut off, that person will be saving himself from death, which is a greater suffering. Shantideva adds that there is another advantage: not only will one be protected from potentially dangerous consequences in the future, but also by experiencing the pain and suffering which has been caused temporarily by others, one is exhausting the karmic potentials of negative karma which one has accumulated in the past. So it serves two purposes.
From Healing Anger: The Power of Patience from a Buddhist Perspective, pages 77-78.