Why are Gheebat and Tohmat Haraam?

They are Haraam because they spoil people's names and characters. When you speak badly of someone, you make others think badly of them. Another reason why it is bad is because the people are not there to defend themselves. If you hear wicked things about others, you should give the others a chance to defend themselves by explaining, before you believe what you hear.
Gheebat and Tohmat are a result of Jealousy. If a person is respected, has done good, has helped others, there will always be people who are angry and bitter that such a person is respected by all. The result is to try and slander and destroy this reputation by sowing seed of venom in their character, by telling the world lies so that the respect turns to outrage and shame. Such people are cursed by Allah (SWT), and are referred to as the evil whispers of mankind.
Allegation is more severe sin than backbiting. It is clear from traditions that anyone who levels allegations against a believer is condemnable.
Prophet Mohammed (s.a.w.s.) declared, If a person levels an allegation against a believing man or woman for an act, which he/she has not done, then on the Day of Judgment, Allah, the Almighty, shall put him on a piece of fire until he is chastised for leveling that allegation.
Imam Sadiq (a.s.) narrates, When a believer levels an allegation against another believer, his faith dissolves just as salt dissolves in water.
Imam Sadiq (a.s.) said further, If a believer accuses another believer (while the latter is innocent), then the honour and dignity that exists between two believers will vanish.
That is why we have been instructed in traditions to restrain ourselves to the best of our ability from thinking evil of a believer.
Rely on the positive points of your believer brother's conduct until you don't get confirmed evidence which prevails upon your existing knowledge of the brother. Do not think evil about the words of a believer in whose defense you can find at least one good fact.
Ameerul Momineen (a.s.) exhorts, Consider the word or action of a believer brother to be good, even if you are offended, and always think optimistically (about him) to the utmost. Do not think bad about him. If you do not get excuse in good actions, search for it over and over again, until the number (of excuses) reaches 70, if you still cannot find it, then think that we ourselves cannot think good about him.
Many times we find that our conversation with somebody about another believer turns out to be completely false. A person asked Ameerul Momineen (a.s.), What is the distance between right and wrong? Imam Ali (a.s.) replied, 'Four fingers'. Imam Ali (a.s.) then placed his four fingers between the eye and ear and said, 'that which is seen by the eye is true and that which is heard by the ear is mostly wrong or false.'
That is why before accusing anybody we should ponder on this fact.
Ameerul Momineen (a.s.) reveals, Happy is that person who is busy in searching for his own defects and is unaware of the defects of others.
Make your intellect suspicious (accuse yourself) of your own defects, for in most cases, self-confidence and self-reliance are the chief causes of mistakes.
In another tradition it is narrated, One who calls himself bad, is saved from Satan's deception.
Just as leveling allegations against someone is prohibited, in the same way a person should refrain from going to places where he may become the target of accusation.
Imam Ali (a.s.) advises: Refrain from the assemblies of allegation and suspicion because the companion of bad people is often deceived by them.
Prophet Mohammed (s.a.w.s.) said, He is more accused of Tohmat (allegation) who sits with the gathering of accusers.
Imam Ali (a.s.) revealed, He who sits in a place where Tohmat (allegations) are leveled, should not blame those who entertain bad ideas about him.
The one who is seen in the assembly of dubious and suspicious characters will find himself a target of accusations and allegations. These traditions highlight that even sitting or being seen with evil people can be harmful for our reputation (even if we don't participate in their evil).
Ridiculing others - a despicable trait: Satan rules when the hearts are constricted and thoughts are perverted. Then man tends to exaggerate even the smallest of matters. Under Satan's domination, he begins to humiliate and ridicule his friends and colleagues. He brands this ridiculing as bravery and courage and prides himself on it. As a matter of fact, he even expects praise and acclaim for this.
Imam Sadiq (a.s.) narrates on the authority of the Prophet Mohammed (s.a.w.s.): O Ali, there is no poverty greater than ignorance and there is no wealth superior to intellect.
According to the prophetic tradition narrated above, the most ignorant of all people today are the Muslims because we have lagged the others in pursuit of knowledge. We have embraced this world as if earning money is the sole objective of life. To acquire money we are prepared to forego religion, faith, certainty and intellect, while the Prophet Mohammed (s.a.w.s.) has made intellect the most superior wealth. Indeed we must reflect on the fact that with the wealth of intellect, the world will be at our feet and we will not have to chase the world.
Imam Sadiq (a.s.) relates from Ameerul Momineen (a.s.), Pride and egotism are signs of weak intellect.
It is a fact that arrogance, pride and egotism may apparently make a man seem very successful in this world, but these very traits are the root cause of his destruction. Pride and conceit are the best indicators of a weak intellect and a person with a weak intellect is capable of initiating a step that can prove ruinous for him socially and/or personally. This is the disease that afflicts most Muslims. We have been split into so many groups and sects because of these evil traits. The soul of Islam has been torn apart due to this and what little dignity and honour is left in it also seems to be waning fast. That is why it is important for us to maintain its (soul of Islam's) dignity and nobility.
The Prophet Mohammed (s.a.w.s.) declared, The most humiliating of men is the one who ridicules others.
These words of the Prophet Mohammed (s.a.w.s.) highlight the fact that there is no place for narrow-mindedness, prejudice and bias in Islam. On the one hand, Islam emphasises the Majesty and Glory of Allah and on the other hand highlights brotherhood and fraternity between the creatures. The level of ethics and morals in Islam can be gauged from the saying of the Prophet Mohammed (s.a.w.s.) when he (s.a.w.s.) calls the ridiculing person as the most degraded of people. That is why as believers and Muslims, it is important for us to respect and uphold the dignity and honour of others around us.
Imam Sadiq (a.s.) exhorts the Shias: Fear Allah! Be a source of ornamentation for us and do not be a source of disgrace for us.
These words of Imam Sadiq (a.s.) not only are an invitation towards goodness and virtue, they are also an indication of the standard of love and Wilayat of Ahle Bait (a.s.). Being Shias of Ahle Bait (a.s.), it is important that we always keep these standards in mind.
The true Shia of Ahle Bait (a.s.) always abstains from sins and abominable acts (makroohat). At the same time, they always hasten towards the obligatory and recommended (mustahabbat) acts. That is why if we truly love the Ahle Bait (a.s.), we must observe the laws of Shariat at all times and try to be a source of pride and happiness for them. With this, even the people of the world will be able to appreciate the true greatness of the Ahle Bait (a.s.).