When He Removes His Help, We Fall
R' Yeruchum explains a Rashi in parshas Shelach (by the Meraglim). Rashi brings down that Hashem said, "I will give them room to make a mistake ." R' Yeruchum explains that Hashem didn't MAKE them do an aveirah - He just took away the help that he always provides, and once that help was gone, it was inevitable that they would fall. *(see bottom for an explanation of why Hashem took away his support).
The Gemara says, "A person's Yetzer attacks him every day, and wants to kill him... and if not for hashem's help, we wouldn't stand a chance." R' Yeruchum explains that in general, Hashem is constantly helping us out, and without that constant support, we wouldn't be able to succeed. No matter how great we are - remember, the meraglim were among the greatest, holiest people, and yehoshua bin nun was listed 5th - implying that everyone above him was even greater than he was! And yet, without Hashem's help, they fell. Yehoshua and Kalev were both saved by an extra dose of Hashem's help, each in their own way (that's another whole discussion).
R' Yeruchum goes further to point out that when a person wants to purify himself (or herself), Hashem "helps" (misa'ayin lo) - but when a person wants to make himself impure, "poschin lo", Hashem just opens the way. Exactly the same point.
He then brings down that R' Yisroel Salanter used to say that he wouldn't trust himself to be in a room with an uncounted box of money unless there were people constantly going in and out. R' Yeruchum said that with his own self-knowledge, he wouldn't trust himself even with money that HAD been previously counted!
He emphasizes that when we go through a day without doing an aveirah, instead of congratulating ourselves, we should realize that it's a wonderful favor and chesed from Hashem, and if we think it's all because of us, we're mistaken.
Why did Hashem remove His help from the Meraglim? Well according to many, if not most commentaries, when Klal Yisroel decided to send the meraglim, they were making a choice that was a mistake of doing too much hishtadlus (obviously, on their own level, which we can barely understand). Since they wanted to do things on their own, midah k'neged midah, Hashem allowed them to be on their own.
May we never be alone, and may we always realize that.
PS - the obvious question on all of this is, why did Hashem make us this way, that we need his help constantly? This is even more puzzling in light of the fact that the whole world was created for the purpose of us having be'chira, free will, and being 'self made'. This bothered me for a long time until I saw an explanation from R' Itzele Peterburger (a.k.a. R' Yitzchok Blazer) - the idea that our constant dependence on Him is to enable us to constantly be involved in asking for help and it keeps our kesher with Him strong. And he explains that we can use this idea in all areas of the physical world to help us strengthen our connection with Him. Whenever we ask Hashem to help us in areas of our physical life - health, wealth, happiness, for my car to work well, etc.