Neglecting the long hours, many PhD programs expect you to have moved from your Undergraduate institution. Likewise, tradition has it that at least one of your two 2-year postdocs should be in a different place. I've been told here that the wider you travel the better. Ideally, you want to have experience in both the US and in Europe, and dabble a bit in Asia if you have the time. Finally, when that professorship comes beckoning, you may not have the luxury of choosing where you work, and may need to move again in order to get that coveted tenure.
But what happens if you meet someone? What if you want to start a family. Well, to put it mildly, you've got a challenge on your hands. Each time the end of a position comes up, you'll have a choice to make: who controls the move? And worse, do you even stay together? This in a nutshell is what is typically called the two-body problem. The resolution of the problem can be a sticky one. But, from what I've seen there are few different solutions which can work.
The simplest resolution is to eliminate from consideration as a wife or husband anyone who does not have a transferable job. Fields like medicine, law, information technology, grade school teaching, and artistic crafts, as examples, are in demand in almost any location where you are likely to find a university. Thus, the academic spouse controls the moves and the spouse with the transferable job follows along. This is the method followed by, perhaps, a little over 50% of couples I know. It works especially well in more traditional families, typically if the academic is male. It also works well when the non-academic spouse is an interested lay-person or enthusiast on the subject that the academic spouse pursues.
But what if your spouse is an academic also? After all, you will typically be working day in and day out with other academics, and there is a certain attraction to shared suffering. In this case, things become a bit more troublesome as academic postings don't come in pairs. Thus you have two ways you can go. First, you can try to negotiate a job offer for a spouse as a part of your academic offer. Almost always, in the rare case where this is possible, the spouse would be offered a position at a lower rank. For instance, one of you would be an Assistant Prof, the other on soft money as a Research Associate. As well, unless your research is "all that and a bag of crisps," you may have to go well down your preference list to find an institution willing to deal. Even if you can make it happen, this can lead to tension as to who is sacrificing for whom by accepting a lesser position. We academics are a competitive breed, after all.
Second, you can do the long-distance relationship, and each of you take your best individual offers. Take it from someone who has been there, that this is not a whole lot of fun. At best, you will need to put off any family aspirations. And at a minimum, you will need to work hard to keep the relationship from falling apart. Sacrifices will need to be made to stay together in terms of time, effort and airfare. Naturally, the closer together you are located the better, and the degree of difficulty goes up exponentially as the number of separating time zones increase. Still, this could be a good solution for those who travel frequently for work. As well, if you prove yourself to be a valuable asset to the hiring department, it might be possible to arrange a spousal hire and a happy ending for all. Even so, this method is clearly a gamble and fails more often then it works out.
So why bother gambling at all? Some chose to end relationships at the time of moving instead. This selection of career over family seems to be especially prevalent amongst more successful academics.
That begs the question: do you have to put your career first in order to be successful? Or can you have both a family life and a professional life? Luckily, more and more institutions are willing to help in trying to balance these two spheres. Flexible work schedules, telecomuting, and spousal job assistance are being offered. Since these institutions tend not to be the top institutions in their field, perhaps they spy that by offering a solution to the two-body problem, they can access a better pool of talent then would be available to them otherwise. I, for one, applaud their efforts, no matter the motivation. After all, a more well rounded and happier set of academics makes for a better learning and collaborative environment.