My "guy friend" can only have for so long, Why?

My ex and i are still really good friends and we still do things and consider ourselves FWB (friends with benefits) We used to be able to go, what seems like forever and now he just cant go that long anymore. Not to say im not satisfied with what we do have but im just wondering why it got shorter all of the sudden...its been almost a year and a half for us. Im kinda curious as to why things changed lately. Is it just that hes tired or that im not satisfying him? Does he want me to do more work? I just dont understand. Can anyone help me?