They will have made contracts with other souls to either continue experiences cut short, step back into dysfunction patterns in order to transform them, contracts for possible long term commitments, etc.
The most immediate purpose for bringing in the "negative" patterns is to identify them in order to heal or transform the "negative" into "positive" frequencies. The most powerful place to do that deeper healing is from within a physical body suit.
We had intended that 90% of "negative" energies would have been addressed and transformed within a system of energy healing cultures. Clearly, that is not currently being done. Therefore, those "negative" patterns intended for early healing are being manifesting into a baby's life very early, within the family, within the religious culture, within the social norms, etc.
When you understand this, it will completely change how you look at the world.
There is no activity of greater value to self and the whole than the rigorous identification and the clearing of old negative karma and removal of outside influence