What about just wanting your own place and your own space where you can get to know your partner and vice versa without other people interfering? Especially in those first years of marriage.
My parents never lived in a joint family system neither did my grandparents. So for me it’s something that I’m just not accustomed to. Also, a lot of guys live away from their parent’s house nowadays due to jobs etc. So more often than not he already has his own place. That doesn’t mean he doesn’t have respect for his parents though.
Personally, although I can see the advantages of a joint family system it just isn’t for me, I’d prefer to have my own home. That being said if I was required to live in a joint family system I’m pretty sure I could get used to it.