I have seen this question asked a number of times, and because I just so happen to be an expert on the topic I decided to make a take on it.
So let's begin with the basics and let me remind you all that players can be both male and female. What exactly makes a player a player? Have you ever wondered why players are the way they are? Were they born this way? Are they heartless? Do they enjoy hurting others? If you do a little research you will find that there are 3 main reasons why players are the way they are and if you think about it it is really quite obvious.
Reason #1: A male who is really gay but either is not comfortable with his sexuality and thinks he has to be straight or knows he is but is afraid of society and being judged. So what does this male do? He plays with females because he feels he needs to prove to everyone that he is indeed straight and loves females.
Reason #2: You have the nice girl/ guy who is tired of being in the sideline. They're the friend, the shoulder that you girls cry on, the girl who you guys treat like one of the dudes. He's the guy who would do anything for you, who truly cares for you and loves you but guess what he gets in return? He gets to see you cry about the men who use you, he gets to hear you call him a "brother" and "just a friend". And guess what he finally realizes after years of failing with girls? That he just doesn't care anymore, that he is tired of caring for someone who just wants a guy who doesn't care about her . I mean it works for all the other guys right? So why should he be the nice guy when all girls seem to want is the bad boy?
Reason #3: You have the girl/guy who has been played before. This is where the cycle really doesn't stop. You get used and played by a player and get your heart broken, you become bitter, you decide that all men/women are the same and in a bitter cycle you decide to play other people so that you no longer get played.
If you look at all of these you will realize that with almost all of these different circumstances the player did once love someone and unfortunately the cycle continues.