In balance I mean if someone is more of a spend-thrift, it's better for them to marry someone who is more frugal. If someone isn't very organized they'd be better off with a partner who is. If someone isn't good with managing money, they need someone who is. If someone isn't that mechanically inclined (like fixing things or putting things together), they're generally better off with someone who is. If someone doesn't have a stable work history, they need someone who does.
No marriage is perfect and all kinds of different types work, but having balance in attributes and skills makes things work more effectively than not. This isn't the lovey-dovey stuff that people in love like to think about but it's the real world. Of course, trying to talk common sense into someone who's in love is like pulling teeth but it's worth a shot.