Yea. but if you look at it that way, look at followers in the real game:
Why does Boone even follow you? there isn't much of a point, I was surprised when I found there was an option to do so. your character asked out of the blue.
Ronnie does not even know you and she wants to join with you.
You also ask Cass out of the blue too.
Raul doesn't really need to join with you for any reason at all, yet he does.
That one doctor guy, the FOTA guy, he seriously does not have any reason to leave that fort, yet you can ask and he does.
Point is that even if you post something back and if you are right, sometimes joining up with people some don't need a reason. The ones in Dead Money were with you and had to watch your back, and you had to watch theirs whether you wanted to or not. If it was me, I think I could start trusting those people. at least I could trust Christina and Dean. Dog... i mean god.... wait.... OK I would not trust him.
Well, Veronica wants to see how different communities thrive in the wasteland and figures the best way to go about it is to hook up with someone who has some wasteland experience.
And depending on how you talk to her she might not join you and if you lose that chance you lose it forever.
But yeah she seems a bit too forward with just joining the PC.
Raul is explained through his story to not have anything at all to go to.
Even if you tell him he's free he just sits there, where he has been held prisoner.
Why? Cause he doesn't have much else to do.
He's basically tired of living.
The PC offers him to join up and even if it at first doesn't mean a lot, it gives him purpose.
Cass' life is basically destroyed.
She had nothing but her caravan.
And when it went, so went her will.
The player gives spark to a fire within her to give her life meaning again.
I mean, what else is she suppose to do? Sit around and drink booze all day.
I don't remember much about Boone, but he lost his wife, he apparently hates Manny for some reason and now his life doesn't really serve a purpose either.
No NCR, no wife, no best friend.
Doesn't leave him with a lot of options.
On the other hand, I don't get why the PC gets the option to recruit him on a whim.
Seems kinda sloppily written to be honest.
Feel free to correct me if I'm wrong on any of these points though, cause this is just how I remembered things.

Anyway, so DoGod/Merge seems... Distant... God only wants what's best for Dog, and Dog is too obsessed with following the Master.
And The Merge, we aren't given enough time to speak with him to give him a reason to stick around for the PC.
All The Merge has is a hunch that the PC did something good for him.
Dean is completely obsessed with the treasure, I've tried to talk to him but I can't find a diplomatic solution.
He just wants the treasure, it's the only thing that makes him want to keep on living.
Even after 204 years he still longs after the treasure.
He is a lost cause, like Gollum from Lord of the Rings.
Christina on the other hand is a bit tricky.
I don't know her full story cause I didn't talk too much to her.
So I can't say whether or not it would make sense for her to become a permanent companion.