To marry somebody is to emotionally depend on them before all others, in perpetuity. Likewise, it is to agree to be dependable for another, as first and last resort. This word “dependence” has a vulnerable quality – without their emotional support we are vulnerable to isolation and collapse from the burden of our emotional lives. We are not social creatures merely for the economy of groups; we socialize to bear the highs and lows of emotional experience and to validate meaning. Parents do these things for their children and partners do it for adults. (At all ages, community is the healthy alternative dependency.) A marriage is a dedicated community of one another. Psychology calls this “attachment.”
The connection or the bond of marriage is an inter-dependence. Emotional dependence is the substance and the life of marriage. You depend on your partner to help you feel what you feel (empathy), know that you are not alone (solidarity) and be assured that your pain or pleasure matters (compassion). In turn you reciprocate these things for your partner. They trust that they can depend on you, that you’ll always be there when emotions arise and that they’ll never have to wait in line.
Without this mutual emotional support inside confident inter-dependence, a marriage is dead, an empty seashell on the beach. It might look pretty on the outside. There may be plenty of other fruitful cooperation, but there is no living connection. Anybody in this situation knows it unambiguously. (Again, broader community is the fallback position.) Conversely, when your partner is dependable you can truly say that they are your “best friend.”