The whole concept of "soul-mates" is based upon having a (mutual) depth of feelings one another. There is a saying: "The person with the least amount of interest in a relationship controls it."
I would prefer to be equally "emotionally invested" in the relationship. Odds are if either of you are not "all in" someone is likely to take advantage of other, take them for granted, or possibly cheat on them at some point in time. Another issue is if the other person concludes you really don't love them as much as they love you it can cause them to feel insecure about the relationship. More than likely their friends or family will point it out to them that you are not are "giving" as much to the relationship as your mate is.