If you’re in a relationship with anyone for long enough, you will have fights simply because conflicts are part of relationships. In fact it is fun to just stand and scream at each other over one common issue that you could have simply overlooked. If you have open communication and no one is afraid of or pretending to the other, your disagreements will definitely come up.
When two people get married, they are merging their formerly separate lives and this cannot happen without some friction. They will have different opinions about so many things and from time to time, this will lead to fights. The key is to know how to fight; with love, respect, honesty, forgiveness and willingness to meet each other half way.
Some issues, however, are recurring causes of fights in many homes and these five are some of the most common;
Money
Money means a lot; security, status, well being, health and plenty other things and people relate to money in different ways and with different values. When two people merge in marriage, aligning and unifying (or not) these money values often happen in the process of many money fights.
Money is a major issue in a marriage and it is often the make or break factor for many marriages.
Sex
Many couples fight about matters relating to affection and physical intimacy, sometimes because of conflicting expectations and sometimes as a fine-tuning process. It’s an important matter and it causes a lot frustration and apprehension and therefore, a lot of fights.
In-laws
Whether it’s about how the husband doesn’t defend and protect his wife enough before his mom and sisters or it’s about how the wife likes to discuss too much with everyone in her family, couples always find something to raise the roof about concerning each other’s family.
It’s been happening for really long time and is not likely to stop.
Children
It could be how one person disciplines the kids or how another is too aloof or too easy on them.
see more here:http://www.nowayobloggers.com/blog/young-families/the-common-things-that-leads-to-fight-in-marriages

I would say among what is listed above, in laws and mother in laws will take number one position

if a man can know how to handle extended families his wife will respect him for that

You are correct... also there are minor issues that can cause argument... like
<<<Television and remote control....ie who watched what
<<< picking scattered cloths, and shoes can be really frustrating for a lady that is a clean freak