Why do guys move on so quickly?

So I’m 21 and my ex is 22. I’ve only dated 3 guys including him. He has date more than 15. And he isn’t really a catch at all. He has no plan for his life, no college degree, works in a factory, and is in a ton of debt. And he isn’t very attractive and I was never sexually pleased with him. But he did have a good personality, but sometimes he was a complete ass. But I think the longest time he has ever gone being single is maybe a month and a half. And I enjoy being single after a relationship ends. It gives me time to process the relationship and make myself a better person. I also date around during that time and it’s so fun because you get to meet all these different kind of guys. But my ex jumps from one relationship to another. And when we first met I remember thinking wow this guy really likes me, but then it became overbearing. I think he has a fear of being alone. And is maybe insecure of himself, and finds his security in a relationship. What do y’all think? I’m not trying to be mean or put him down, but I want opinions because maybe I’m wrong and maybe he just is able to turn off his feelings really fast. I mean we dated for 7 months, and talked about getting married just a few days before we broke up. I don’t know, opinion?