Relationship Breaking Point

So, I haven't been in many serious relationships that have lasted long but a lot of my friends have. Right now I'm just dating this guy and I started realizing that most of my friends have broken up with their partners at the 8 month mark. Why do they suddenly start having tension around this point? I get that the honeymoon phase is over and you don't need to impress them anymore but all of them started arguing more 8 months in. What happens?!