For me it always seemed like a business deal. You decide to become partners with someone and invest in each others success. My wife for example bought me books on programming early in our relationship, now that I have applied those skills and got a job in the field the extra money I bring home benefits us both. She is rightfully entitled to some of that ROI should I leave her because she did a lot of the initial investment, a legal marriage only formalizes that agreement and encourages it.