The personnel concerned are nearly always people who have just come out of the college and assume that because they give a specific clearance the aircraft will always be able to comply with it.
Indeed. I would summarise the issue thus:
"The problem with pilots is that they think that their aircraft is the only one in the sky.
The problem with controllers is that they believe that the mid-air collision is the only hazard faced by aircraft."
A little more understanding of the big picture on both sides would be helpful.