Reincarnation
I'm not well versed enough in Buddhism to know exactly what the schools of thought are on reincarnation in the religion, I just know that it is generally a thing.
I've encountered the idea that reincarnation is a choice to some degree. The choice being the type of life that one is reincarnated into. I'm thinking that this is more associated with Edgar Casey and New Age thinkers than Buddhism, but if I'm going to talk about reincarnation this seems like a good place to do it.
What I mean by choice is that the soul of a person actually chooses the kind of life it will live in the physical realm before it is incarnated and also in line with karma from the previous life. This idea seems to imply a radical personal responsibility for a lot of what happens to people in the physical realm while there is also an element of free will while incarnated.
What got me going in this line of thought is that I've had a pretty difficult life. I have a nasty form of bipolar disorder that induces psychosis. At the same time, I've worked like a mule in the market since I was 16 years old. We're talking 30 years, and that continues to this day. For the past 4.5 months I've put in 66 hours a week. An ordinary week for me is 55 hours. I've worked harder in the past.
The particularly venomous bipolar disorder is enough to sideline most people, but I'm one stubborn s.o.b. I've had it since I was 20 and I've worked through it- the first 10 years untreated. I've often wondered from a karmic perspective why my life has been so difficult. Now throw in the idea that I've actually chosen the circumstances of my life before I was even born....
I guess I could no longer complain. This is mostly what I ordered up. Go figure.
= new reply since forum marked as read
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 
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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.
