This "nirmatsara" quality is derived from the Supreme Lord Himself actually - because He is equal to everyone, even though we think that why Krsna seems to be favouring the other devotee but not me?
In 7.1.9:
"The all-pervading Personality of Godhead exists within the heart of every living being, and an expert thinker can perceive how He is present there to a large or small extent. Just as one can understand the supply of fire in wood, the water in a waterpot, or the sky within a pot, one can understand whether a living entity is a demon or demigod by understanding that living entity's devotional performances. A thoughtful man can understand how much a person is favoured by the Supreme Lord by seeing his actions."
In the purport, Srila Prabhupada states:
..."A teacher instructs his students if the student is capable of taking more and more instructions. Otherwise, in spite of being instructed by the teacher, the student cannot make strides in his understanding. This has nothing to do with partiality. When Krsna says tesam satata-yuktanam bhajatam priti purvakam/ dadhami buddhi yogam tam, this indicates that Krsna is prepared to give bhakti yoga to everyone, but one must be capable of receiving it. That is the secret. Thus when a person exhibits wonderful devotional activities, a thoughtful man understands that Krsna has been more favourable to this devotee."
This is not difficult to understand, but envious persons do not accept that Krsna has bestowed His favour upon a particular devotee in accordance with his advanced position. Such foolish persons become envious and try to minimise an advanced devotee's position. That is not Vaishnavism. A Vaishnava should appreciate the service rendered to the Lord by other Vaishnavas. Therefore a Vaishnava is described in Srimad Bhagavatam as nirmatsara. Vaishnavas are never envious of other Vaishnavas or of anyone else, and therefore they are called nirmatsaranam satam."
Gurudev says before we criticise a devotee or anyone about anything and in that process attempt to throw him into a hole, we have to dig that hole ourselves. When we dig that hole to bury him with criticism, we do not realise that we are actually in that hole first before he got in!
Your servant,
Devakinandana Das