Step-by-step explanation:
Suffering is a choice that we are unaware of having made. We can choose not to suffer only if we can become aware that we are choosing to suffer, and when we are capable of choosing not to. For most people choosing not to suffer is not an option.
Consider the suffering caused by the loss of a loved one. There is pain when you suddenly lose someone important to you. This pain, just like any other pain, will overwhelm and dominate one’s experience, and will dull one’s awareness significantly. It’s hard to think clearly, or at all, when one is in pain, let alone be deeply aware of oneself.
Even if you’d want to try to be aware of yourself when you are in pain and see if maybe you keep the pain going where you really don't need to, there is a slew of cultural norms and beliefs that require you to suffer when you lose your loved one. If you don't suffer it means you are heartless, emotionless sociopath who is incapable of love. You are going to be judged very harshly, by yourself and others, if you are not in deep enough pain.
It would require a considerable presence, strength of commitment, drive and motivation to dig your way out from under the pain you are feeling, the scrambling effect this pain has on your mental faculties, the social pressure that demands that you suffer and your own beliefs about how you should behave in certain situations that require you to suffer. Most people don't have that level of presence, strength, commitment and drive so, even though technically they have a choice, they are not capable of choosing.