When's it time to quit?

How do you know when it's time to call it quits with someone?

I have a friend who has been in a loving relationship for several years. Many people regard this pair as a model couple. They mesh well, have a great time together and have similar interests. However, one of them is a little more ambitious than the other. Now the ambitious one is questioning whether they're an actual good fit.

I don't necessarily see this as an end-all, be-all to the relationship, as long as both parties continue to bring equal contributions to the table. Maybe one lives a stressful life but brings home more money and maybe the other provides solace for all that stress. Assuming they get married and have children, one person could stay at home with the kids, which is definitely worth the contribution in my opinion.

However, this is something that the more ambitious person has got to accept. If the more ambitious person starts to resent the less ambitious person's ways, that will only hinder the relationship.

Just a side note, the less ambitious party does have a job, but doesn't seem to have career goals in mind, something that the more ambitious person has had since a teenager.

What should my friend do? Do you think it's possible for ambitious and not-so-ambitious people to be together? How do you know if the person you're dating in a perfect match, and is it necessary to be a perfect match? How do you know when it's time to call it quits?