I think the problem with the phrase "soul mate" is that like the phrase "one true love" it's probable origin is a description of the side-effect of the biochemical dopamine and it's tendency to cause the delusion of "salience" when people fall "in love". It's a universal phenomenum that when people say they are "in love" their brain scans show certain activity and they report that the love object is "special" or "unique" and their relationship is "meaningful". It would probably be better to think something like "I freely choose to make you my soul mate." which might mean "I will choose to exhibit behavior that will cause you to be the only person with whom I will allow myself to get wasted on 'in love' chemicals"" than "You are my soul mate." which implies a certain dependency on fate and the whim of the love object. Therefore, I would further posit that it would perhaps be best to choose a love object/subject with the same qualities one might seek in a drinking buddy and/or tennis partner.
"Tell me, what is it you plan to do with your one wild and precious life?" - Mary Oliver
