Seforim say that due to your reason of people accidentally sinning new sins while coming back as a gilgul to solve an issue from a previous life, most people in the world today are gilgulim.
In regard to every person having a tafkid to accomplish in this temporary physical world. For some people it takes a full 120 years to accomplish their mission and others I personally know have left the world as infants as early as weeks to months old cause maybe their only mission in life was to bring happiness to the world due to in their previous life they gave someone pain so they needed to return to this temporary physical world to give happiness to their new family or others just for a short time and Hashem chooses who their family should be based on many things including only a family that can handle the test of faith of chas Vshalom losing an infant child so young. Cause in truth we all are sent to this temporary physical world to be tested by Hashem and each test you pass brings you closer to Hashem and also immediately brings you to the next test ex. As soon as you finish the test of faith of finding a shidduch you immediately start working on the tests of having children and Shalom bayis tests etc…..