What's mine is mine
Many couples, both married and otherwise, live perfectly happy lives together without ever sharing a bank account. If both partners work and have independent incomes, this makes perfect sense. Once shared priorities are dealt with, mortgage, rent, utilities etc, then each person is free to spend whatever they agree is left over, as they see fit.
Mistaking money for love?
However, many people see sharing a bank account as a symbol of their 'togetherness' and so the issue can become emotionally loaded. If one person is careful with their money, their partner sees that financial caution as meanness and it can affect their relationship. Generosity, on the other hand, is often mistaken for love.
If you are determined to share your bank account with your partner, It's a good idea to establish some ground rules. As you're likely to have very different spending habits and priorities, both of you will need to learn to compromise and meet in the middle. What your partner decides to buy for themselves is their own affair, but not if it's making a hole in money you jointly need for shared costs, like paying the mortgage. Both of you will have to adopt a mature approach and realise that a shared bank account is a not a green light to spend someone else’s money.
An alternative might be to keep your joint account for important 'shared' expenditure like mortgage, holidays and so on, but maintain individual bank accounts for 'personal' spending.
The bottom line is, sharing a bank account is a statement that you trust the other person in your life implicitly. But it does put pressure on both of you to respect one another and be responsible about your spending. Sharing is caring!