A major theme of פרשת כי תבוא is the concept of שמחה. After one brings his first fruits to the Kohein in the Beit Hamikdash, the Pasuk says, ושמחת בכל הטוב , that one needs to rejoice for all the good that Hashem gives us. There is an entire chapter in ארחות צדיקים devoted to the subject of שמחה. It is important that one sees all of the blessings that Hashem has given him and he should show appreciation by being in a state of שמחה all of the time. Negativity, bitterness, and self pity, prevent a person from being both appreciative and happy. In that chapter on שמחה, the ארחות צדיקים adds that often we are frustrated when our good intentions are not noticed. The fact that Hashem sees everything and HE appreciates what we do, should also bring one Simcha. As Rav Nachman said, מצוה גדולה להיות בשמחה תמיד, that it's a Mitzva to always be in a state of joy!