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I observe that people can go through years of psychological therapy to try to untangle the complicated web that results in a particular problem. And I can say for my self that I seldom have a clearly defined and unitary motivation for my decisions.

Both my posts and those of numan present a reason why collective decisions tend to be less complex than those of individuals. There is no hard and fast way to determine the process that leads to complexity.

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SO, what puzzles me is how you can see uncertainty in one set of complexity... and yet with laser like precision see through even greater complexity and arrive at certainty as to the single unifying motivation.
I don't think that it is with "laser like precision" but I do think that there is sufficient evidence to say that the hypothesis of economic motivation has more than a fair chance at being correct.

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Many people get married. the choice of partner will likely have an economic impact on their future. But it does not seem a fair conclusion to say that the choice of marriage partner is almost entirely driven by economic factors.

The hole in your logic is that, again, you equate the individual to the collective decision. And, what is more, your example is perhaps not the best one, given that there are many examples where economic gain IS a main reason for marriage.

"The liberals can understand everything but people who don't understand them."
Lenny Bruce

"The cleverest of all, in my opinion, is the man who calls himself a fool at least once a month."
Dostoevsky