If you review your previous love stories you will remember that you have liked many people from the opposite sex, but only those who appeared similar to your character, to your values, thoughts and opinions, have managed to pass that normal point of likability and caused you that deep attraction which often results in love and obsession.
Similarities that might cause you to like someone don't lay only just in your personalities, many often the fact that you both live in the same city or you both attend the same course in your university might force your mind to think unconsciously that you are very similar to each other even though you can't understand the reason why you feel so.
Feeling similar to someone intensifies the initial attraction because every person has an inner desire to feel safe and secure with his accompany. Similarities between people make also possible for them to adapt their lifestyles easier with each others lives.
However researches have pointed out that in addition to similarities, some differences must be present too, in order to make that relationship work. Different personality traits make people's relationships interesting and satisfies their desire to find compensation for their weak points in someone else.
Such compensation wipes their feelings of helplessness that come from their weaknesses.
Similarities and differences are usually felt in deeper stages of relationships. Unknown people can form only a general impression about each other that might be positive or negative, but while they know each other better and while they spend more time together they reveal more similar feelings and traits in each other and this fact increases their feelings of attraction and strengthens their relationships.
This topic is widely covered in one of the chapters of the book How to make someone fall in love with you which is the only book that talks about falling in love by using real world datas and researches.
Relationships are very complicated and no one can pretend to explain everything about them even by writing ten books, but we can touch the most important aspects of it for sure. I will add many other articles in my category Relationships explained to show you even more interesting facts.