This is why men come back

He sees that you are happy without him, and his ego is damaged. This will often cause him to reach out in hopes of trying to dictate your emotions to strengthen his ego. 

He notices that you appear to be doing better and may believe that you are beginning to move on. This belief prompts him to reach out in fear. When you successfully move on it'll make it harder for him to return. 

He's testing you to see if it is indeed possible to abandon you and return whenever he wants to. He's priming you for an extended rollercoaster of emotions. If he knows that he can hurt you, disrespect you, neglect you, then leave and decide when he'll return, then he'll most definitely continue to do it in hopes of continually breaking you down. The more you let him back in, the harder it is for you to move on. 

He's either bored with the person he initially left you for and or  the reason is no longer interested in him. When men leave all of a sudden, it is usually because they've taken an interest in someone new. Here's something a lot of women don't know. Men use a relationship as a point of ownership, and even when that relationship ends for whatever reason, that man will add your name to a list of names of women he's been with. He will then reference that list to see who he still controls emotionally. You remove your name from that list by refusing to allow him back into your life. 

You are rare, you're magic. Leave that mediocre shit alone! 

In some cases, a man returns because he regrets losing you, but it's important to remember how and why the relationship ended in the first place. It makes no senses to allow him back into your life when you've spent most of the relationship uncertain as to how much you mean to him. You're reading this for a reason, you're here in this moment with me for a reason, and i hope you understand that you deserve so much more. I want you to be happy, that's all