As you know, there are different schools of Buddhism - and they will answer differently. Also, American (or 'western') practitioners distill Buddhism differently that in the 'East' - so people on this board (or others here in the US) will answer this according to a 'western' interpretation of the practice.
Here is one answer
.
There is an understanding of suffering, that suggests that we can carry suffering for others. One story of Kwan Yin is that she (sometimes, she is thought a he - depending on tradition or country
But that she takes on the suffering of the world. She welcomes it. This practice is a gift of compassion. I know many people who practice in meditation and sit with this. They wish to become like Kwan Yin and take on suffering.
But keep in mind, it is not simply to sit with the grief or suffering, but rather to breathe it in and then release it.
Because even suffering - your suffering, is impermanent.
Most people come to Buddhism looking for an answer to your question - why do we suffer. It's a great question to bring to meditation. In its smallest form - it's why me (why, oh why, oh why.... me). In it's larger form - it's what is the reason we all suffer. Why is there suffering in the world.
Your question asks about karma and about the choice to suffer. Do we make this choice and reincarnate to a life with suffering?
Wake up! (I am hitting you.......can you tell?)
Wake up in this moment. Connect, clearly with this moment and see through your suffering to THIS moment and your question leads you to clear mind.
Or, this question will become very small and just about 'small I' or the ego. Grow yourself to Big I (the world) and you will become like Kwan Yin - a true Bodhisattva.
Does this make sense?
You must sit and practice and become this answer.