Should we save or go on a vacation? How come you spend money on yourself but I haven't spent a thing of myself because I'm buying child care? And then of course the tension can get even worse when it's a blended family situation with his and/or her and maybe even our kids.
The joint account is funded by income into the family from whatever sources. Each partner may or may not have income sources but if they do, a couple decides what percentage of the household income is brought in by each partner. That percentage then determines the amount each partner is responsible to put into the joint account. The joint account services common living expenses such as rent/mortgage, food, childcare, utilities and other items the couple may agree on, such as vacations together.
More discussion here about the impact of money on marriage/relationships and what to do about it: