It's a nice world they built.
It's just the game is so exhausting. Rockstar's design philosophies just start to wear thin as the game goes on. The constant start-stop of the main narrative/gameplay thread has always and still feels awkward. I'm still asking the question "Why am I doing this? Why is this happening?" and struggling to come up with an answer. It's a better game than GTA4 for sure, but it doesn't seem to care about keeping me interested in the main thread to the end.
Right now, I find that their reasoning are much more sound than CJ or Niko's. In this game, they're all career criminals with miserable lives. Michael wants to be a father but also wants to do crime, he's also in need for cash; Franklin is a very detailed character, and seemingly does everything in the game up to this point to win the heart of his old lover, and since Franklin's idea of what being a good man equates to having money, money, money as he tries to buy the love of Tanisha; Trevor is Trevor.
The fact that the jobs RARELY pay adds to a lot to this. Now, they're in want of cash, but they already HAVE cash after the first job, but they're at the point of the typical human error of always wanting more. This theme is excellently written into Michael's character. At the beginning of the game, he practically has everything - the family, the house, the cars - and yet he thirsts for more. He gets what he wants, but he doesn't stop. He wants MORE.
When Dan Houser said that GTAV is about machismo and being a man, I think he meant it. Notice how the strong female characters - Tanisha for example, or heck, Michael's wife - are completely correct in their portrayals of their men.
I'm not at the point where you are yet, and there's definitely a bit of "why am I doing this?" but I noticed that every time I ask that question, it's a Trevor mission. So there's that.
Unlike GTAIV, I think Rockstar has done a fantastic job of showing how endearing, and yet how circular the psyche of its leads are. With Niko, he always talked about needing money, and no more killing, but he'd kill some guy for 2000 bucks. After Three Leaf Clover, he still does it. There's very little motivation given as to why. But the three in GTAV? The game, so far, gives ample reasons why they still continue to do it. V has managed to be a far more coherent critique on the American dream than IV ever dreamed of. That aside, the one problem is one of the long length game. Almost every long length story-based game falls into this loop. It happens every story-based Final Fantasy - except VI and X, maybe II - where the original mission of the party diverges, and the game just turns into what the fuck. GTA suffers from the same shit.
I did the Trevor cargo plane mission, and I was wondering what the entire point of it was until I realized that they're riffing on themselves by taking a mission that was similarly in San Andreas, but showing just how fucking STUPID the entire concept is by having the air force bomb you out of the sky. Nice touch, while at the same time, allowing a little mayhem. It's nice that they're actually self aware now, which is refreshing.