What do you think is the main reason for a unhappy marriage?

Misunderstanding is usually on infidelity. Finance is the life blood of modern life. So this can cause problem. In laws or anyone who matters can cause obstacles. But if you have proper understanding, there will be no obstacle and all issues will be settled. Hence, ultimately, misunderstanding is the major cause for unhappy marriage.

I think, misunderstanding is main thing. But sometime Financial status also a reason for a unhappy marriage.

Unrealistic expectations and misunderstandings among the partners are one of the main causes for an unhappy marriage. Family issues in the form of trouble some in-laws are the secondary causes...

ya im also agree with the option Misunderstanding...This thing break the pure relation...

There can be many other reasons apart from the ones you have mentioned,
Forceful marriage against one's wish etc

All are the reasons for an unhappy marriage.But misunderstanding is the main reason for it.

Other reasons for unhappy marriage could be lack of love, too much expectations, immaturity of the married partners and other outside influences like social, economical and parental...

well misunderstanding could be a reason but even lack of love is also a reason for an unhappy marriage....

Recently, one more cause of unhappy marriage has been revealed. This is nothing but a mobile phone. Many young wives love the phone morethan anything else. This is annoying by itself. Moreover, the phone enables the newly sed wife to commmunicate to herparents even the slightest matters and invite undue interference.

Yes I agree that mobile phone is also a cause and has its influence to effect married lives...