Why did the first dog in space die, but humans didn't

Laika likely died within hours after launch from overheating possibly caused by a failure of the central R-7 sustainer to separate from the payload.
Humans didn't die because humans have the ability to diagnose and solve problems - dogs can't.

Well the reason they sent animals in space in the first place is because it was very experimental, and they didn't want to risk human lives. It's possible that something was wrong with the spacecraft. Dogs are also going to be a little more fragile than humans, so it probably couldn't handle the g-forces associated with space travel.

The experiment with Laika was just to see if an an animal could survive and operate in space. There was no plan for her to survive as the engineers hadn't figured out how to return the capsule safely yet. After she had proven that yes, an animal can survive and work up there, the plan was to adminiter drugs to put her sleep before the oxygen ran out. As it happened, a failure in the temperature control system caused her capsule to overheat far sooner than expected, and she died from that. Her capsule eventually burned up when its orbit degraded and reentered the atmosphere.

Hmmm ...
The dog was sealed inside a small sphere. There was some capacity to replenish oxygen for a few days. So when the dog breathed in most of the oxygen it would have suffocated. But there also was a problem with the cooling system in the capsule and the temperature rose to 40C. The dog died about 5 hours after launch on Nov 3, 1957. Though the exact cause of death was not given.

Certainly there was no plan for the capsule to return to the earth's surface to recover the dog safely. Over five months later, after 2,570 orbits, Sputnik 2 disintegrated—along with Laika's remains—during re-entry on April 14, 1958.

The intent was for the dog to die. The capsule was not capable of re-entry, when the life support ran out, the dog died a suffocating death. Truly a horrible experiment. The humans went up in capsules designed for re-entry so they could return to Earth before their life support ran out.

Because having Laika actually surviving the entire experiment was not a priority. They just wanted to find out if a living thing could go up into space without dying just from being in space.

Well if you are talking of the dog that died yrs ago and is still orbiting, then they have shorter life spans, didnt provide it with nourishment, the dog wasnt planned to return from space; Was planned and it was used as an experiment all in the name of science. Im pretty sure there is a chimp in space as well...