Why is marriage so tough??

Why is marriage so tough. Why is it so easy for spouses to hurt each other?
I have discovered that marriage is one of the toughest things on the planet. Maybe it is just my marriage that is so tough but I don't think so. My wife thinks so low of me that I stand little chance of ever pleasing her in any way. In her mind I am just a massive burden that she regrets accepting. This is really depressing... I'll stop now and try again tomorrow.....