Jgnat hit the nail squarely. It's not rocket science.
I performed 3 marriages. All three are divorced. I once made a tally in the margins of my Watchtower (study on Marriage) of all the weddings I could think of attending. As a witness you go to a lot of them. Half were divorced.
Once when Karl Klein was visiting my parents we were discussing the problems with the failed 1975 date. I mentioned how so many of my peers got married because they were afraid that if they died during the great tribulation they would never experience sexual intercourse. The result was a sunami of broken marriages. Klein said he would have to add that to the list of causalties of 1975.
The big mistake is that the Watchtower teaches you that prayer, study, and communication will solve all your problems. They pay little attention to temperaments that just don't get along. They should at least allow couples to divorce without consequences if they are mutually agreeable to that solution.
A lot of times the problems begin right after engagement. The mask comes off and people start showing their real personality instead of their courtship facade. So what clever rule does the society come up with? They declare you imature if you break an engagement and even take away congregational positions. Losing your responsibilities is viewed as a loss of Jehovah's confidence so you go against your intuition and marry setting the stage for a miserable life.