Rabbenu Avraham ben HaRambam, in his work Ha'maspik Le'oved Hashem, writes that the highest level of bitahon is what he calls הפקדה, whereby a person gives over all his wishes and desires to Hashem, saying, "Whether or not I receive what I want, it doesn't matter, because I know that everything that happens is Hashem's will."
The more we love somebody, the happier we are when we see that person's wishes being fulfilled. Therefore, the Or Ha'haim writes in his Hefetz Hashem that a person with complete love of Hashem views himself as a vehicle through which Hashem's will is fulfilled, and is therefore happy under all circumstances, no matter what happens.
The Yam Derech cites the Netivot Shalom's comment that if a person feels content with his lot in life, and satisfied with whatever Hashem gives him, then Hashem will, in turn, be satisfied with what that person achieves even He expected more and the person has fallen short of his potential. Hashem rewards this person מדה כנגד מדה- since he is always satisfied even when he had wanted more, Hashem is likewise satisfied with what this person has accomplished. Accepting the way Hashem's treats us is very powerful and a great service to Hashem.
The Midrash, in Parashat Ki-Tisa, relates that when Moshe Rabbenu was in the heavens to receive the Torah, he saw all the magnificent rewards that await the tzaddikim. He then saw one particular area of rewards, and Hashem told him that it was reserved for those who care for orphans. Moshe then saw a different area, and was told that it was designated for those who devote their lives to kindness. This continued until finally Moshe saw one area with extraordinary rewards that far surpassed everything he had seen until then. Hashem told him that this was a treasury of מתנת חנם - "free gifts" that will be given to those who do not necessarily deserve great rewards.
The obvious question arises, why would the greatest rewards be reserved specifically for those who are not deserving of special rewards?
The Bayam Derech answers that the Midrash perhaps refers to those who are always content with what they have, and always accept the way Hashem treats them, even when they do not receive what they want. Even if technically they do not deserve the great rewards that Hashem had hoped they would deserve, nevertheless, מדה כנגד מדה, they will receive it as a מתנת חנם.
If a person has been out of a job, this is an opportunity to achieve greatness by saying, "Hashem, if this is Your will, that I should be unemployed at this time, then I accept it and will be happy, because I love You and Your will is being fulfilled." If a person is waiting to find a marriage partner, and is feeling despair, he or she can achieve the highest levels of bitahon by saying, "Hashem, I surrender all my wishes and desires to You. I am happy with whatever You want from me right now." This is a very significant and powerful way of serving Hashem, and can propel a person from the lowest depths to the greatest heights in but an instant.
We should train ourselves to always realize that no matter what happens, it is all a manifestation of Hashem's will, regardless of whether or not we understand how this is. And by recognizing and accepting this truth, we can reach the highest levels of greatness.