It is an elusive and subjective term and any definition would be to apply a generalisation to personal relationships. It could be argued that searching for a 'soulmate' will only lead to frustration because it is questionable that such an entity even exists.
As a well known song states "It's got to be, perfect and I won't take anything less". Mary follows this statement too closely and is lead to constant disappointment, because she believes life is like a romance novel.
But for people who actually have a brain, the idea of "soul mates" is ridiculous. There are certain people who are right for you at certain times in your life. There isn't ONE person who you are always supposed to be with forever.