For me it is the fact that there is no division of labour. When you're a lone parent, everything is down to you. Every decision, every chore, every food shop, every meal, every load of laundry, every arrangement/logistic, every pile of dishes, every bag of rubbish to the bins, etc.
Or that might be just me at the moment....we're going to visit my family for a month tomorrow, so I'm in laundry/packing hell...not to mention clearing the fridge, etc. And on top of that, both bathrooms are being completely gutted/redone while we're away, so everything must be completely cleared out and packed away.
Was just thinking earlier how much easier/faster it would be with another pair of hands, and someone to joke with about it all.....not to mention, someone to help pay for it!