It is said that the angel Gabriel stands in the Upper World, and as couples in our world get together and have children, he sends down the souls to be born.
The soul itself chooses where to be born—its parents, its environment, even the difficulties and challenges that will allow it to make the corrections it needs to make—so that it can accomplish the most spiritual growth in life.
Life in all its convoluted forms brings us to one purpose: to fulfill our job in this world, to take the karma we were born with and change it into something more positive and beautiful, to take the original gift of life we were given and to eventually bring it back to the Creator, whole and complete.
To learn more, we recommend reading
To Be Continued... Reincarnation & the Purpose of our Lives
by Karen Berg