Disagreements about spending money. Even if you'd agreed on a budget, there can still be issues on how to spend money and what you will put your savings toward. Many, many marriages cite financial troubles as a legitimate issue in their marriage. In order to help deal with this conflict proactively, you can agree on a financial system before your finances merge.
It is normal in a marriage that there will be some “personality clashes”. Consider the fact that the spouses are two completely different and unique people who have experienced different backgrounds and upbringing. Now they are living together and seeking to forge one unified marriage. It is a huge adjustment and it will take time to smooth off the rough edges through the day to day challenges of keeping house, earning a living and relating to each other constantly. Personal circumstances can add to the stress, for example if there is sickness or a health issue, pressure from other family members or a demanding work environment.