I believe this is because marriage is a lot of work. It's a responsibly, an agreement, a pact between two people, and it takes a lot of energy, both physical and emotional. When things are going well, it's a beautiful bond unlike any other. When it's falling apart, it can feel like a complete devastation of who you are as a person.
This doesn't mean that marriage should be perfect. It would be unreasonable to expect that two imperfect people with differing backgrounds and personalities would never disagree. Healthy disagreements can even be good for a marriage. But no spouse should ever feel like a failure.
I recently read a blog post that broke my heart. It's from the blog of a married man living with narcolepsy. In this particular post, he discusses how narcolepsy has created an insurmountable barrier between him and his wife. The post made me teary because I wondered how often my husband felt that way without me knowing it.