Okay, so what is a soul? Energy? Spirit? A combination of the two?
If we surmise that we exchange the most energy throughout our interactions with people closest to us then we can also surmise that we become linked or entangled with them. One of the people doing research in this area through the study of psi is Dean Radin. Basically, a person with whom one is entangled is better able to sense emotions or physical condition at a distance. (paraphrasing and prolly not explaining very well. Check out his blog or his books.)
So if we exchange energy with those closest to us (and others but I'll just stick to this example for now) and a soul is spiritual energy then souls may possibly become entertwined with one another. If we are exceptionally in tune with another person then we may find what we need within them; within their energy. This may be a balancing force and not necessarily a matter of personality. I read somewhere, and no I can't remember where, that a transference of energy has to 'grow back' (again paraphrasing) similar to resting to regain energy, eating to regain energy, replenishing blood lost through injury or transfusion, or in this case restoring spiritual energy or 'soul'.
If we share energy through friendship, love, sex, or what have you and the mind, body, and spirit are all interjoined holistically, then the energy must also infuse the other person. Even down to a cellular level. Since cells renew themselves and we are not the same person (cellularly speaking) from day to day, then that 'dose' of others energy must be renewed as well.
I think this might be why most therapists and counselors subscribe to the tenet "Know thyself." If we get lost in -only- that energy of another and lose sight of our own wellspring, codependency could result. However, an in tune relationship could, as the addition of instruments to a symphony, become more than the individuals involved, blending and creating harmonics and overtones.
Chakras are meant to be representations of energy as they traverse the body. From the Red root chakra up through the violet or white Crown representing peace, higher spirit or personal soul. The colors represent different areas of spirit and body and as the frequency increases they move upwards towards the spirtual. Color is also used frequently to balance a deficit, much as a fully balanced orchestra can be an incredibly rich, lush sounding thing. The frequencies present in music create harmonics and spiritual energy (frequencies) merging and blending could create overtones in those as well by analogy. (All frequencies blended create white, or the divine, higher spiritual color)
In the end it is up to us to find those we 'click' with. To blend our souls, our energy and try to make something better both within and without ourselves. To teach and learn, to love and be loved.