In the laws of the Chanuka lights and court testimony, husband and wife generally have the status of being the same person. From this, Hilchos Derech Eretz observes, in all areas of life (other than religion, the domain of the man; and the home, the domain of the wife) the husband and wife are joined in a state of union.
I once told a marrying couple that if one hurts a spouse, one hurts oneself, weaken's their bond and undermines the "team." If one benefits a spouse, one benefits oneself, strengthens their bond and builds the "team." They are one. They are the same organism. They "sink or swim" in the "sea of life" together.
If your teeth decide they want to pick up the pencil that dropped and your right arm decides it wants a chance to chew food, your overall body is going to work less efficiently. Different organs have different roles. There's no value judgement or discrimination. It the way nature gets things done. If it ain't broke, don't fix it. To best serve the overall interests of the body, the teeth keep chewing, the hand picks up pencils. If your right hand accidentally hits the left with a hammer, does your left hand take the hammer and smash the right to get even? The same body is caused pain! A couple is a body. There has to be unity.
There is another insight to be gained from the principle of: a husband and wife are as one person.
The Mesilas Yesharim writes that every single moment of life is a test from Heaven, given to the person for the purpose of passing. Consider this in our marriage context. On a surface level this means that one always has to consider one's partner. Every moment, one has to view one's partner as a test and opportunity. In every moment and on every occasion one must regard and behave towards one's partner so as to pass the test of every moment and situation.
Going deeper, consider that each thing one does (at each moment) one's use of that moment - choices, actions, etc. - have an impact on one's soul. If one passed the test of the given moment, the impact is good. If not, the impact is not good.
Married partners have impact on each other. Constantly. If, in marriage, A chooses or does something that has an impact on his or her own self, that impact - even though it is on oneself - has impact on the other. There is no such thing as deciding something based on it having impact only on self. If the impact is negligible or innocuous, or if the impact has any impact that doesn't bother the other, OK. For instance, if a wife wants to study knitting in her spare time, the husband needn't necessarily care, especially if it makes her happy.
But if a partner does something that allegedly only impacts him or herself but that impact can impact one's partner, this could be wrong. This goes against the grain of democracy and modern ideology (which is of no concern to the Torah when a violation occurs). If A's impact on self can impact B, A must not be selfish or blind to the impact on B.
If A wants to enter a profession that hardens people (e.g. lawyer or New York City Parking Violations Bureau), the prospect of becoming a hard or nasty person is not for A to decide alone. If there could be, even in the future, an impact on B or on the marriage, A cannot demand fulfillment or self-expression. The use of A's moment to decide to go into something that can have an impact means that A can fail the test of that moment of life when deciding to do something that can hurt or shortchange A's partner. To be continued.