This post is dedicated in memory of Etya Sarah bat Yitzchak ha-Levi. May it be an aliyah for her neshama.
Doubt: it's an uncomfortable feeling, which can manifest itself in a number of ways. For example, many of us have felt that moment of dread, where you ask yourself, "Did I turn off the stove/iron/lock the door/etc." It could also show up after you find the clarity to quit a job, take a new one, finally lose the 20 lbs., or to propose to your girlfriend, or dump your boyfriend. In one moment you feel so sure of yourself, and in the next, you are doubting yourself and wondering if it's really a good idea. Even in situations that we are fairly certain, we still doubt ourselves. Doubt isn't a new phenomena, invented in order to sell anti-anxiety pills and apps that let you check if you closed the garage door. It's been around since the beginning, or at least the beginning of the Jewish people. Taking a look at Sefer Devarim, it appears it exists to remove doubt reassure the Jewish people that the right thing to do is to follow Torah.
Much of this week's Torah portion, Eikev, is spent reassuring the Jews that if they follow the mitzvot, they will have a great life. Moshe goes out of his way to highlight a few transgressions in particular to refrain from - worshiping false gods or money; attributing one's success to themselves and not G-d, the sin of the Golden Calf. Moshe also reminds the people that even though for forty years their clothes and shoes didn't wear out and they always had food and water, the Jews still freaked out every now and then and doubted G-d. Much detail goes into everything good that will happen when the mitzvot are followed, especially in the Land of Israel. There is also a reminder of what can happen when the Jews forget (such as when they freaked out Moshe was late and built the Golden Calf).
The second paragraph of the three paragraph 'Shema' that is said every day is also written here. By now you can probably guess the theme of the paragraph, even if you've never read it: essentially to remember and do the mitzvot every day (going so far as to tell the Jews to put them 'between your eyes' and 'on the doorposts of your home'. Those are some well-placed reminders!
So what is it - do G-d and Moshe not think the Jewish people can remember a few rules? The fact is, when it comes to doing something correct, but perhaps less fun that what everyone else is doing, it can be all-too-easy to convince yourself that the fun-but-not-kosher activity is really ok to do. Of course, it is only okay if you doubt the validity of the Torah, or the repercussions of what will happen if you decide to do your own thing. So, G-d makes sure to reiterate these things many times. It's not that the people are stupid, but it is part of human nature to second guess yourself. This doesn't make people bad or weak, just human. It might be natural to doubt, but it's just as natural to overcome these doubts and remember what is right.