All of the answers above are top-notch, but another, more literal reason exists. I apologize if this is long. The Dark Side's natural philosophy that Vader abided by is what made him allow Sidious to convince Luke to kill him, and it's what made him stand at Sidious' side as he electrocuted Luke. Vader's submission to Sidious caused him to see his fight against Luke as a test to prove his strength to Sidious. And Luke Luke pretty much also saw it the same way even though he was their enemy.
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remove commentary, it's not a long answer :)