Agreed.
No 1 reason: People don't really know each other when they get married (it takes years to know a person)
No 2 reason: People change all the time, and at times we change very fast
No 3 reason: Expectations. Damn expectations. Mostly on the part of women, fairy tales expectations of being saved by the prince charming and being happy ever after. Yes, WOMEN BELIEVE THAT CRAP because they are conditioned to believe it from early childhood.