This may not be a good idea, but I am tempted to put this in computer programming terms. My feeling about this as a native speaker who just saw this question come up on the sidebar is that when you say "the seasons", you are passing a data structure, whereas when you say "seasons", you are passing the data individually.
So if you want to "understand birds and bees" = understand (birds, bees), your mental procedure might be to understand birds then understand bees. Anything relevant about the two individual concepts should be understood, but you don't need to have any particular reason for focusing on those two organisms.
But if you "understand the birds and [the] bees" = understand ({birds, bees}), now your mental procedure looks at the data structure passed to it {birds, bees} and notices that there are exactly two items on that list, and might consider what they have in common, and look in a table of idioms and observations about that particular list of elements.
Similarly, "understanding seasons" means you learn something about summer, then maybe winter, then you might look at fall... you might understand something about each season. But you haven't defined them as a structure like {spring, summer, fall, winter} that has a certain number of entries, perhaps you think of them in a specific order or at least specify them as repeating, and you try to understand the common basis of the phenomenon rather than the basis of each season.