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why you go that way since this is the nearest way ?
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Since means A causes B. We write "A since B" or "I'm happy since you're happy".

Conversationally, this sentence is understandable (don't forget "do" or "did"):

"Why do/did you go that way since this is the nearest way?"

Grammatically, these sentences are better:

"Why do/did you go that way? I ask you since this is the nearest way."

Good luck.
Comments  
Thank you very much, ryan!