Finding your life partner is in some sense like doing a numerical approximation. After the first a hundred steps if you still cannot find a satisfactory approximation to the true value, you're probably better off stop and try with another algorithm. Despite that there might be a "perfect" match for you in these some seven billion people, but since one's life-span is finite, we can only meet finitely number of people from which we choose the most suitable spouse among them.
Assume you are a male (for the convenience of narration only; all the results can be similarly applied to a female reader. However, due to the radical difference between marriage markets for hetero- and homosexuals, this model may not well apply to the latter group.) in your twenties; you have met some 300 similarly-aged women whom you have contact with throughout these years. Let's further assume that 280 of them do not qualify your life-partner criteria, and ten of the twenty left are separated from you geographically or otherwise physically unreachable. Among these ten, five have already been married off or are currently engaged in a long-term relationship. Therefore there are only five candidates, from the initial 300, that meet your criteria and is geographically available.
Okay, now since most people have similar judgements over a "fit" couple, the five candidates tend also to find you somewhat suitable too. Therefore the chance of engaging with one of them should be sizable. Mostly likely the first one among the five that initiates / accepts a partnership with you will eventually make it to the end of this life's most extensive filtering process.