I do not think that in established successful democratic countries Prime Minister or President interfere with army promotions. Promotions, transfers, and delegation of responsibilities happen according to system within the institution, and that is institutional own affair. Government should only give tasks to institutions.
Actually, such interference only happens in corrupt failed democratic countries (like Pakistan), where corrupt person in power are scared of institutional heads, and wants to keep hold of their political offices regardless of corruption, nepotism, incompetency, misconduct and mismanagement.
In Pakistan interference was same in every institutional heads. But judiciary became strong and politicians scared of judiciary, so now senior most Judge is getting elevated to the post of Chief Justice after retirement of current CJ on 12 December. Same should have happened in army … but no, corrupt PM like to interfere with promotion of Army Chief so that he can feel safe in office along with his corruption.
Politicians have introduced such wrongs in constitution, not for benefit of country, but because of self-interest. Such promotions create grudges within institutions’ top hierarchy (including army) and that to happen is obvious. People working in any institution would not like that outside person (like Prime Minister) interferes in their promotion to top post due to his partiality, insecurity, self-interest, nepotism, or any other reasons. Most efficient and hardworking people working in any institution would like to see that merit, seniority, and hard work is observed.
Corrupt politicians in power love such interference to make sure that person of their liking gets appointed at top posts of strong institutions, who becomes obliged to them for the post, and thus do not act or interfere against their corruption, nepotism, incompetence, misconduct and wrong doings.
Anyhow, such interference always backfires at one time or another, as it creates grudges, disrespect, and hate in institutions, especially in people at top hierarchy of the institutions but at lower level too, towards politicians.
Even one who gets promoted to top post feels he got promoted in violation of merit and seniority, but his promotion is due to partiality, insecurity, self-interest, corruption, and nepotism of PM.
Only incompetent and low character Army general (or head of any institution) who may have used connections throughout for his promotions, could feel good getting promoted to Army Chief in this way. PM also do that considering that the one he promoted must be of low character and would feel obliged to him and would overlook Pakistan interest if he (PM) do anything for self-interest harming Pakistan’s interest.
Most in Pakistan armed forces throughout their career get promoted on merit and seniority, so seeing such bias promotion to the post of Army Chief, most in army must be feeling bad (even Army Chief who benefitted).
For instance … after promotion to Army Chief, in any private meeting, it would be difficult for ‘Gen Raheel Sharif’ to face ‘Lt Gen Haroon Aslam’ or ‘any army officer’ knowing that in fair circumstances where merit and seniority is respected, he (Gen Raheel Sharif) would not have been Army Chief but it would have been Lt Gen Haroon Aslam. If meeting happens, than it is also possible that (in joke or seriously), Haroon Aslam (or any army officer) may ask Raheel Sharif that how much he paid (or what he gave) to corrupt PM for getting elevated to Army Chief surpassing him (Haroon Aslam). [Obviously, serving officers may not ask such question, for obvious reason]
In return army (and Army Chief) lose any respect for such PM, and do not hesitate to kick him out if he does anything wrong.