> Knowing the terrible fate to himself and his soldiers if he surrendered to the Soviets, why did Friedrich Paulus surrender to the Soviets?

Friedrich Paulus did not technically "Surrender", he and his staff was captured by the russians. In either case, it's not like he had a choice in the matter beyond shooting himself (which is what Hitler tried to not-so-subtly tell him to do by promoting him to field marshall at the very last moment and at the same time reminding him that no german field marshall had ever surrendered). Paulus was a catholic and believed suicide to be a mortal sin, he also forbade it among his men along with intentionally getting shot which he considered the same thing.

Continuing the fight in Stalingrad was not an option. The sixth Army was finished, spent and and depleted. An army only on paper. The fact that he held out encircled for three months is remarkable in itself. Paulus knew Hitler had wanted him to kill himself which might in part explain his later cooperation with the Soviets. Imagine you had done nothing but follow orders, and done so very well, then you're asked to go kill yourself. I'd be a little pissed off and Paulus absolutely was, telling fellow captive General Max Pfeffer "I have no intention of shooting myself for this Bohemian corporal".

> Follow-up: When Paulus survived the war and moved back to Germany, how was he treated by veterans?

Paulus returned and settled in East Germany after the war (in 1953, 2 years prior to the general repatriation of other German prisoners). He died only 4 years later and to my knowledge had no remarkable interactions with veterans in that time.