"For the time will come when they will not endure sound doctrine." Now they will still listen and willingly receive both teaching and admonition. Later you will no longer find this in them.
"The holy Apostle prophesies and, foreseeing this in the future, commands that the flock be taught truth and virtue with every effort and without weariness. Thus, instructed in faith and trained in good works, they will later remain unchanged when others turn from truth into error" (Ambrosiaster).
The Apostle does not indicate the reasons why this will happen. Yet from what was said before, one may suppose the reason will be failure to fulfill the command given, that is, to teach persistently. It will be the silence of pastors and negligence in preaching truth and instructing the flock.
Pastors do not teach, and the flock remains ignorant of sound doctrine. They do not admonish sinners, and sinners become hardened in vice.
Remaining ignorant of truth, they gather empty thoughts and every kind of nonsense, and because of this become dull in receiving truth. Hardened in vice, they become incapable of accepting admonitions and reproofs.
Therefore one must hasten to enlighten them with sound teaching and accustom them to virtues, before they become dull to receiving the former and hardened against submission to the latter. For when they come to this point, it will be too late to take action. They will not listen.
But since they cannot remain without knowledge and rules, what will they do? "According to their own desires they will choose teachers for themselves." That is, teachers who will leave them to think as they please and live as they wish.
It is noteworthy that "they will choose" is, in Greek, επισωρευσουσιν. It means they will gather a whole heap or crowd of teachers, and they themselves will gather them. Of course, only those pleasing to them will be accepted, while the displeasing will be rejected.
The result is that these teachers will depend on those taught, and will teach not according to their own program, but according to theirs.
"Nothing can be more expressive than this word επισωρευσουσιν. It signifies a disorderly crowd of teachers, who will even be ordained, chosen and confirmed in their rank, by the disciples" (Chrysostom).
"And in this choice they will act not according to sound reason, but will choose those who indulge their desires and say and do only what pleases them" (Theophylact).
"Having itching ears." They will act thus because they have become accustomed always to hear only what pleasantly tickles the ear. This points also to eloquence, but even more to flattery or adaptation to the listeners' tastes.
They will choose only those fit for this, "seeking for themselves people who would speak for their pleasure and flatter their hearing" (Chrysostom).