I'm not sure what you're asking for here, 'the German perspective' in what sense?
The German army made some serious strategic errors in their attempt to 'take' Stalingrad most of which I might add were fueled by Hitlers arrogance in his own military ability, a mistrust in his own Generals' ability and his petulance in wanting to take Stalingrad as quickly as possible to 'teach Stalin a lesson' and break the will of the Russian people.
The Germans used their usual 'blitzkreig' tactics of hitting the city with Panzer divisions and infantry supported by the luftwaffe but made the fatal mistake of almost bombing the city into oblivion which hindered their later progress of clearing the city of Russian resistance which basically took too long.
The Russians (what was left of them) dug in and fought a guerilla war in the bombed out remains of the city which meant that the two most powerful parts of the German military machine, the Panzers and Luftwaffe could not be deployed. The harsh winter weather played its part in effectively cutting off the German supply lines and they had to be increasingly resupplied by air which wasn't practical in terms of the weather.
This allowed the Russians sufficient time to move other Russian armies into position to encircle the Germans both inside and outside Stalingrad thus cutting off any hope of the trapped German armies inside Stalingrad from being reinforced or evacuated.
The Germans made too many mistakes and underestimated the resolve (and sheer quantity of potential reinforcements) of the Russian army.
If you want any one particular reason it was Hitlers arrogance and over-confidence.