Each of us defines love differently as well as have our own expectations of how one "in love" behaves. In truth there is no "right" or "wrong" way to express love. There is only "agree" or "disagree".
When someone is unhappy with their mate it is because they aren't being loved the way (they) want or (feel) they should be loved. Not everyone expresses love in the same ways. This is why (mate selection) is so important!
If you or the person you are with has to change their core being in order to make the other person happy then it's possible you have selected the wrong mate for yourself. Ultimately everyone wants to be loved and appreciated for who (they) are.
Having said that it is possible to fall "in love" with someone who is NOT "in love" with you. If someone states: "I love you but I am not (in) love with you." They are essentially saying: "You are not (the one)".
The underlying definition of "soul mates" is two people who share the same values, want the same things for the relationship, and naturally agree on how to obtain them. Last but not least they have a (mutual) depth of love, desire, and respect for one another. In order for him/her to be "the one" they must see you as being "the one".
There are only two ways to experience joy and peace of mind in relationships: we either get what we want or we learn to be happy with what we have.