Reincarnation is the process used by Spirit to progress the soul through evolutionary cycles. With each lifetime, the soul repays karmic debt, receives rewards for karmic credit and sets-up situations that cause the soul to face spiritual lessons and make choices to progress the enlightenment of the soul. If we learn our lessons, we move forward, if we repeat old patterns and chose not to learn, we can remain stagnant and will have to repeat the lesson again.

What is the spirit?
The spirit is the culmination of all our lifetimes, energies, and spiritual essence. The whole of who we are. Every life, every memory and every action ever taken on ones spiritual path.

What is the soul?
The soul is a subset or smaller section of our spirit. The section of our spirit that we chose to pull into a single incarnation to work with and work on. Of all the lifetimes we've had, we couldn't cram all of the karmic debt/assets or lessons into one incarnation. It would be over whelming and too much to work on in one life. So we choose to re-incaranate time and time again.
The composition of who we are, our eye and hair color, skin tone, family genetics and so on are wrapped in the DNA we get from our parents. This is the physical DNA.

So what is included in the Spiritual DNA?
Other than the karma brought forth from those past lives, we pull in characteristics to define our who we are as an individual. These 7 layers or 7 spiritual characteristics which also make up who we are. All these characteristics are combined with the physical DNA make you the person you are today.
Our Origin/Purpose
Our Talents (such as communication),
Our Skills (such as a skilled sculptor),
Our Environment (issues associated with living in a big family for instance),
Our Back Ground or Creed (such as an Irish Catholic),
Our Potential (such as spiritual awareness)
Our Spiritual Balance (such as incarnating as a female when most of your previous lives have been male.)

Who chooses which lives we bring into this one?

Before you're born or attach to the physical body your going to inhabit in this incarnation, your spirit makes a few decisions. You decide which past lives to pull into this one, you decide the lessons and karmic events you'll work on, you can make an agreement with another soul(s) to join together in this coming life to resolve personal karma. All these issues are defined in a Master Plan (like a blue print) for this coming life.
As with any blue print, as you build the structure, issues or situations arise that cause you to make choices to further detail or refine the original plan. This is the law of Free Will, this is how we progress and evolve the soul. Our spirit can provide the situations, but the final outcome is never predefine. That is determined by free will and choice, that is how the soul learns and hopefully progresses along it's spiritual path.

Why find your past lives?
Sometimes people are just curious about who they were, what they did and what happened to them. But there are many things you can learn about yourself, this lifetime, how the past affects your spiritual purpose in this lifetime, what karma you have brought into this embodiment, or why you do some of the things you do now. Sometimes you can address issues you have in this life, by understanding why you do those things, or why you're facing certain issues or events. But you have to find it first. You can do this yourself through meditation, but it's not an easy process which is why past life regression therapists can help you. Not only guiding you through the regression process, but also talking with you before and after and helping you find your life path and move forward.