I would agree with you that the first sentence is a generalization. However, the generalization is not used in the way described by answer choice E. Instead of support for the conclusion, I would describe the first sentence as background information.
Try to stay focused on the conclusion, which is this: "But people should not necessarily try to rid themselves of this tendency."
The support for this conclusion is not the previously mentioned generalization, but rather the sentences that follow. The key word here is "analogy." That is what the author is doing with the parallel line example. This is such a dumb analogy that it is difficult to follow. However, we still have to take bad arguments seriously. The author is basically saying, "fixing one tendency would be unreasonable, so fixing another tendency would be unreasonable as well." This is best described by answer choice C.
If answer choice E were correct, the argument would look something like this:
People tend to make certain cognitive errors when they predict how a given event would affect their future happiness. Therefore, Jimmy should not get married, because it probably won't make him as happy as he thinks it will.
This is using a generalization to argue against a course of action.