It is easy to know what mate is right for you. From the psychological and biological perspective, read Blink of Malcolm Gladwell just to give you an idea how our brain works.
There are other theories about how to find the right mate for you and the bookstore would be such a fertile ground for tips on how to find the right mate.
However, there are biological and psychological explanations that you would need to consider. Aside from this, you need to consider the social aspect of the relationship.
If you read Blink, you'll know that people know whether the person is right for them. People are 95% correct when they were asked to score couples who would have lasting relationships and those which will end up either in divorce or plain break-up.
Sometimes, our brain has functions that we tend to neglect. Rationalizations are sure way to convince yourself that one person is the right for you. But it only takes a matter of seconds or minutes for you to recognize that an area of the relationship will fail.
Articulation is difficult. But intuition is not a matter of the heart but of the brain.
Of course, you might say, "love is more complicated than that". But that is exactly my point. It is not. Life is simpler. Human beings complicate it.