It can not be denied. Souls travel together in numerous lifetimes. In some lifetimes souls are in love with each other, and sometimes they fight and kill each other. In doing thousands of regressions it has become clear to me that the souls really do love each other -- at the spiritual level. They enter into various relationships on the Earth plane to help teach each other and to learn valuable lessons. In the end (Whenever that is!), all the lessons are about learning unconditional love.
This ongoing long journey makes sense when looking at the ultimate big picture of multiple reincarnations leading to enlightenment. It even makes sense when looking at connections of souls between a past life and the present life. Not only does the existence of soul mates make sense, the cleaver manner in which the soul mate pops up in a lifetime at the right time for a lesson to be learned is more than amazing! And it is far more than "coincidence".
There is some evidence that one of the souls may skip coming into a lifetime in a physical body with the other soul mate, and instead stay in spirit form. It appears the spirit form soul mate acts as a "guide" or "spirit helper". These spirits never interfere but do help when asked, -- but that is a whole other area I won't cover here.
From my personal experience in doing numerous Past Life Regressions, the reason two souls are together again was due to one promise or commitment made during a powerful emotional experience in a past life. Here is one example:
It appears soul mates need not remain together for long periods in a lifetime in order to fulfill the intended purpose. The relationship is not always loving, either. In one case explored in a Past Life Regression, a soul was born as a son to a soul mate mother in a past life. The son died approximately age 6 in that lifetime. The purpose of their relationship was for the son to help the mother learn about love and releasing her son without resentment at God.
There is a belief that souls get together in their spirit form before birth in a physical body and agree on how they will interact and for what purpose in their next lifetime. It appears that this planning is to help the souls on their karmic path toward enlightenment. The agreement may even be for one soul mate to kill the other. But this opens a huge area of discussion about karma and predestination, and that is beyond my purpose here.