Spring sat when Stampy and Lee were building the Playfull Po Po Station. While Stampy and Lee were building the Police Station, Stampy decided to check on Spring to make sure he was ok, then Stampy went back into the unfinished police station, which lead to fortunate consequences. At around 16:42, Lee digs a piece of dirt, then Stampy hits Lee, forcing Spring to attack, but fell into the pit. At around 16:52, Spring falls into the water. They left him in the water for 21 seconds, then at around 17:14, Stampy heard Spring whimpering, but doesn't see him. Stampy thinks Spring is getting hurt by a googlie, but he is not. Spring is actually nearly drowning and then at around 17:18, after he nearly drowned for 4 seconds, Spring died as a clear result. Stampy tries to let Spring teleport. But, Spring didn't teleport. So then, as Stampy returns to the Police Station, he noticed Lee's head is down, telling him that Spring drowned. Thinking that Spring didn't drown, he decides to find him. Stampy calls out Spring's name and he said "If you're there, make a chicken noise." As Spring did not make the noise it is instantly assumed that he died, but because his corpse or experience points could not be seen, it is possible that he might have teleported away, and never came back to Stampy, who gave a melancholy and heart-breaking speech about how Spring was still with him.
After his death, Bengy became the Police Dog in honour of Spring.
It is possible that he could have committed suicide because Stampy didn't let him help build the Po-Po station, He leapt into the water because he was fed up with sitting down and not being able to roam about and play.
As said in the Death and Legacy paragraph, Spring may have been taken by Hit The Target and tamed by him, so now Spring is on his side. However, his grave is still there so we are not be 100% sure what happened to him.