Every kindred spirit you fall in love with is in a relationship with someone else, attracted to a gender/sex configuration that isn’t yours, or very seriously only wants to be your friend. Some people will not neatly fit into a category, and will instead just string you along.
Every person that falls in love with you is either someone you only want to be friends with, is not the gender/sex combination you’re looking for, you don’t feel a romantic connection with, or otherwise expresses interest at the very times that you’re already invested in someone else.
To add further to the paradox, many people that would be good fits for one another never actually hold conversations or speak up about how they feel. Also, there is no formal system of courting.