[based on a dvar torah written by R’ David Silverberg]

Parshat B’haalotcha contains several unfortunate incidents that occurred after Bnei Yisrael left Har Sinai:
In Perek 11 the Torah recounts the incident of Kivrot Ha-ta’ava, where Bnei Yisrael complained about the mahn and demanded meat, and were subsequently punished.
Next, in Perek 12, Miriam and Aharon speak improperly about their brother, Moshe, and they are punished with tzara’at.

Rashi explains that these two stories are juxtaposed because one led to the other; Miriam only learned of her brother’s divorce as a result of the events of Kivrot Ha-ta’ava. (In response to the nation’s demand for meat, Hashem instructed Moshe to appoint 70 elders to assist him. During the formal designation of these leaders outside the camp, two other men – Eldad and Meidad – began prophesying within the camp. Tzipora, Moshe’s wife, observed the prophecy and expressed her concern that Eldad and Meidad may divorce their wives as Moshe had divorced her. Miriam overheard Tzipora’s comments, and it was then that she spoke with Aharon about Moshe’s divorce.)

Rabbenu Bechayei, however, suggests a connection between the two incidents, not necessarily because one led to the other, but rather due to their resemblance. Both incidents involved “hotza’at diba” — negative speech about something or someone special. In the episode of Kivrot Ha-Ta’ava, Bnei Yisrael spoke disdainfully of the mahn, claiming that they were better off with their diet in Egypt. Rather than appreciating the extraordinary miracle of the mahn, which fell each morning from the heavens and sustained every member of the nation, the people complained about it. Similarly, Miriam, rather than speaking admiringly and reverently about her brother, found something negative to express. Just as Bnei Yisrael complained about the miraculous food Hahem provided them, Miriam likewise complained about her brother, the greatest prophet that ever lived.

Rabbenu Bechayei adds that the next narrative – the sin of the spies, in Parshat Shlach – continues (or perhaps culminates) this theme of hotza’at diba. (The Torah specifically refers to the spies’ reports as “dibat ha’aretz” in 13:32 and 14:37)  The spies chose to focus their attention specifically on the challenges they viewed with settling Eretz Yisrael, rather than on the Land’s remarkable qualities and innate kedusha. The story of the Mergalim thus continues the topic of looking negatively upon the great gifts given to us by Hashem. 
In all three episodes, a person or a group of people sinned by finding fault in a great person (Moshe), place (Land of Israel) or thing (the mahn).

These three complaints perhaps reflect three broader areas where this mistake of hotza’at diba is often made. 
1. Kivrot Ha-ta’ava — Material discontent and dissatisfaction with financial conditions: The mahn represents a gift from Hashem, which is how were are supposed to view everything we have, and whatever that lot may be, we are supposed to be satisfied and grateful.

2. Miriam and Aharon’s lashon hara — Inclination to comment on the behavior or shortcoming of others: It’s always easier to evaluate someone else instead of ourselves. If we look hard enough, we can find fault in anyone – even Moshe Rabbenu. Our goal should not be to point out the negatives in other people, rather to focus our attention instead on admirable qualities.

3. Meraglim — Focusing on the challenges and complexities of Jewish destiny, rather than its opportunities: The spies had the privilege of being the first ones to go into Eretz Yisrael, and instead of relishing the opportunity of planning how to settle the Land, they focused on the difficult challenges that this privilege entailed. The catastrophe of Cheit Ha-Meragelim reminds us to keep our attention focused on the great opportunity we are given as Hashem’s chosen nation, and to approach the challenges that go along with this lofty stature with enthusiasm and confidence, rather than excuses and complaints.

All three instances demonstrate the dangers of negativism. A negative, fault-finding outlook prevents us from feeling satisfied with our material blessings (Kivrot Ha-Ta’ava), damages our relationships with family and friends (Miriam and Aharon), and discourages us from actively pursuing the spiritual goals and national duties that are demanded of Hashem’s special nation (Chet Ha-Meragelim). 
If, instead, we are able focus on the positive qualities about everything and everyone around us, then we can more readily find happiness, fulfillment and pride in our daily lives and our service of Hashem.