Siri Bhagat Kabir jee has written the following Gurbani Pankiti:
ਨਿੰਦਉ ਨਿੰਦਉ ਮੋ ਕਉ ਲੋਗੁ ਨਿੰਦਉ ॥ ਨਿੰਦਾ ਜਨ ਕਉ ਖਰੀ ਪਿਆਰੀ ॥
(Slander me, Slander me O people Slander me. Slandering is very dear to the Servant).
Someone asked as to why Siri Bhagat Kabir jee has praised Ninda and why is Ninda so important for Gursikhs. Here's what was written on this subject sometime ago:
Why did Bhagat Kabir jee want to get slandered? Here is what comes to the mind but these are just conjectures:
1) Ninda keeps one's haume in check. When only Ustat (praise) happens, there is a chance that one's haume would get inflated. True seekers of Vaheguru care not about Ustat or Ninda. They know that both of these actions are happening in this false world and for this reason have no bearing on the actuality of things.
2) Ninda makes one vigilant and one is more careful of making mistakes.
3) Ninda happens because of some bad Karma. Ninda is a cheap way of burning one's bad Karma. This world is a false place i.e. not real. The slandering that occurs here has no bearing in the real place i.e. the Dargah. 'Nindak hamre kapre dhoye' literally means that the Nindak at the cost of incurring sins, washes away our sins.
4) Many times, when the nindak crosses limits, the Gurmukhs take refuge in the Naam of Vaheguru. It prompts them to japp Naam with more vigour. So Ninda being a form of dukh (sorrow), acts as the fuel to burn the lamp of Naam. Gurbani says: ਨਿੰਦਕੁ ਗੁਰ ਕਿਰਪਾ ਤੇ ਹਾਟਿਓ ॥ ਪਾਰਬ੍ਰਹਮ ਪ੍ਰਭ ਭਏ ਦਇਆਲਾ ਸਿਵ ਕੈ ਬਾਣਿ ਸਿਰੁ ਕਾਟਿਓ ॥੧॥ Nindak stops with the kirpa of Guru Sahib when Vaheguru cuts off the bad habit of the Nindak of doing Ninda.
This above is what comes to mind. There could be more reasons too that other Gurmukhs should post. What was in the Sarbag (immortal) mind of Bhagat jee, only they know. We are making guesses only.
More thoughts would be welcomed.