The meraglim told Klal Yisrael about the giants who lived in Eretz Canaan. They said (13:33), "We felt like grasshoppers, ונהי בעינינו כחגבים, and that is how they perceived us, וכן היינו בעיניהם."
The Midrash states, "Hakadosh Baruch Hu forgave them for saying 'We felt like grasshoppers,' but Hashem didn't forgive them for saying 'that's how they perceived us, וכן היינו בעיניהם.' Hakadosh Baruch Hu told them, 'How do you know which impression I had them think of you? Perhaps they thought you are angels?'"
This is a crucial lesson. Many people have a low self-image and think everyone looks down on them. Other people's opinions are very important to them, and when others think negatively of them, it hurts them immensely. But one never knows what others are thinking. Perhaps Hashem put into the hearts of others to look up to you and to consider you special. Maybe they even think you are like a malach. Why do you think so poorly of yourself?
Thinking good about yourself is essential because this will encourage you to act in a proper way. If you think lowly of yourself, you will likely fall to sin and bad middos.
The Chidushei HaRim zt'l explains that the negative image of the people in the desert was their primary sin, which caused them to sin with the meraglim. Therefore, Moshe told the meraglim, והתחזקתם, "strengthen yourselves..." Moshe understood that they have negative self-esteem, and they have to strengthen themselves and think better of themselves.
The Sfas Emes asks that the Gemara (Sotah 35.) states that the meraglim heard the Canaanites say, "We see people; they look like ants, crawling in the vineyards." It seems that it wasn't only the meraglim's imagination. They actually heard the Canaanites looking down at them, thinking of them like insects.
The Sfas Emes answers that the problem began with their negative self-perception. This influenced others to look at them. "Since they were small in their own eyes, and they consider themselves like grasshoppers, this caused others to see them that way."
This happens often. The way one views himself is what he portrays to others. Since they considered themselves small, this caused others to see them as small.
Had they believed in themselves, the Canaanites would have respected them more. Perhaps they would even consider them like malachim. But they looked down at themselves, which is how others viewed them, too.