Giving something you love up entirely for someone else is incredibly hard (I did it) and resentment is a dangerous bedfellow to that sacrifice. If one is honest with oneself, and chooses to make the sacrifice, then one can never, never resent the other for it - it's dishonest and will ultimately lead to greater failure. Honesty with oneself and the other is key.
A whole lot of one's perspective on this comes from where they are. JT - I seem to remember a long time ago you mentioning that flying cost you a good relationship - so I can totally see your point.
There are others of us who come to it from the opposite side - not being relatively supportive of each other's careers and passions was one of the many items (it's never one thing) that killed my marriage...we never could find a middle ground on certain issues and eventually we quit trying. It was the best thing for both of us.
I'm in a relationship now where we have that middle ground - she wants to pursue a MFA degree (she's already got one MA) at some point in the future, and I still want to develop flying as a career, so we've been making plans that will accomodate both sets of goals. It may take longer, but it's really about the journey. Of course, it helps that her Dad was a CFII, too.
It really comes down to the two people involved - some people can handle a certain level of sacrifice, others cannot, and if you can meet somewhere in the middle, then you're probably going to be okay. Optimism helps a lot. We can make all the blanket statements we want, but it's safe to say that just like choosing a place to train, it depends on what's right for you.