For me, it's mostly about the friends I've made. Lately it's been a bit of a grind, since me and a few others separated from previous FCs to start our own and had to rank it up and get the gil for a house (and then had a bit of a letdown when the plot we wanted sold JUST as we got the rank and money D: ), but we got a house just this week and things got a lot better with the fun bits of decorating and not needing to craft/spiritbond/etc to get points and money.

I was part of a group of players who started a linkshell for people tired of the endgame "exp only! speedrun only! loot only!" mentality, and we filled up the last spot just the other day XD So many fun people to talk to, and almost always people willing to help you out when you need a dungeon/primal run. Except Aurum Vale That always seems to make people run for their lives, for some reason.

Still haven't downed T5 yet (due to never having found a full group willing and ready to learn it with me), so my FC's next goal is to recruit people who mesh well with us, both skill-wise and personality-wise, and get a static that way.

Back when the game launched and all the way to February or thereabouts, this game was pretty much just a Co-op between me and my flatmate. It wasn't until we met the people who we started that linkshell with that we began to socialize more with others, and it became a completely different game from then on :O Back then, I said (after Titan Story mode) that I'd never do any HM Primals. The horror of it all! But now, I actually think the HM Primals are fun! (Even Titan!) And I've even dared to join other people in TeamSpeak!
I don't think I could go back to Co-op anymore. XD

TL;DR version - The people make me look forward to playing almost every day. Sorry for the wall of text