Judging from the situation of the United States, I'd safely state that holding Mars too tight (if possible at all, but that's rather tech-related, thus irrelevant here) would cause an independence war. Fair treatment is a key, but also depends a lot on local power - which is also depending on more factors, like tech level, weaponization, how organized and united they are, and so on. African kingdoms were easily defeated in the colonization ages.
I'd consider enormous distance as a factor as well; with communication and personal meetings rare enough, Martian (or any colony's own) culture would quickly take a much different turn, and history taught us that different cultures might be (but not neccesarily are) problematic, causing conflicts. Hell, just try to imagine global Martian corporations!
Specific reasons might also be created to stay together. Everything's true - and their opposites so are their opposites. Might be politics, economy, or other interests too. It might even be forced, or otherwise out of necessity, constraints and need. I'm saying it because even if water, oxygen and food are solved (which are, well, yes, vital), other products might be traded between Earth and Mars.
These are the spare thoughts I could add out of the water.