Governments love couples. It's slightly more reasonable than individuals; it's a step closer to better demographic (and without the hassle of importing people); it's just the ideal foundation for stable society. Tax authorities have a wide range of policies and measures to support and promote this ambition. Lower tax brackets (averaging income disparity), higher tax breaks, specific exclusions, all are benefits targeting couples. Tax burden can be significantly lowered for two individuals when filing as a couple, and consulting with tax professional is generally worthwhile. Some countries even have specific allowances on top of tax reliefs.
Unfair? Maybe... but today Effy focused on taxpayers' efficiency.
All things considered, taxation is only one tiny example of all potential economies of scale specific to couple life. Lower rent / person. Lower living costs / person. And it goes endlessly.
So psychological reasons aside, economy alone is enough for anyone to consider couple life. What about love in all that? A chemical stuff... maybe.
"Honey... tax season is around the corner, so come and bring in your big fat invoice..."