When one gets attached to his or her significant other and settles down, it's not merely a union of two person. They love each other, double their joy, share their sorrow, and offer comfort in each others' company. It is also a synergistic link where the couple is able to compliments each other's quantities and bring out the best in themselves.
Just like how when you work in a partnership over a project with another person, where there is opportunity to bounce ideas between each other, and their ideas may just be the missing piece(s) (and vice versa) to a blockbuster of an idea, just as how mixing blue and yellow paint gives you different shades of green.
On a more tangible level, it could be a simple thing, such as pooling up resources to invest in stocks with a lower commission, as opposed to buying twice. It may give you the ......